Color separation into a plurality of ink components including primary color ink and spot color ink

ABSTRACT

Utilizing an ink set that includes a plurality of chromatic primary color inks that, when used in combination, can reproduce achromatic color, and at least one spot color ink of hue different from any of the plurality of chromatic primary color inks. Color to be reproduced on a print medium according to any one input color is termed “reproduction color”, and the combination of ink quantities in the ink set for reproducing the reproduction color on the print medium is termed the “color separation ink quantity set.” The ink quantity of the spot color ink included in the color separation ink quantity set is adjusted in accordance with a lightness parameter value which is correlated to lightness of reproduction color, so as to reduce ink quantity at a rate of change greater than the rate of change of the lightness parameter in the direction of increasing brightness.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/773,087, filed on Feb. 4, 2004, the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a color printing technique employing aplurality of inks.

2. Description of the Related Art

In recent years, color ink jet printers have come to enjoy widespreaduse as image output devices. A typical color ink jet printer employs, inaddition to black (K) ink, several kinds of ink of the hues cyan C,magenta M, and yellow Y. Any color in a color image can be reproducedusing these several kinds of ink.

In a printer of this kind, ink amounts of each useable ink aredetermined according to an arbitrary color of the color image. Herein,the process of determining ink amount of each ink used in printing forcolor reproduction in this manner will be referred to as a “colorseparation process” or “ink color separation process.” Relationshipsamong color data of a color image and ink amounts of each color arepre-stored in a color conversion look up table (LUT); during printing,ink amounts of each color at each pixel position are determined withreference to the LUT (see, for example, JP10-191089A).

Color reproduction by a particular printer is determined by theparticular types of ink useable by the printer. Typically, any color canbe reproduced by combining three chromatic primary color inks (e.g.,cyan C, magenta M, and yellow Y). In some instances, spot color inks ofdifferent hues from these chromatic primary colors ink are used. “Spotcolor” refers to a color that can be separated into two chromaticprimary colors. Where spot color inks are used, the range of colorreproduction can be expanded. However, to date, in instances where useof such chromatic primary color inks and spot color inks is possible, nofeature has been devised whereby color separation may be performed inconsideration of image graininess caused by spot color ink dots, or inconsideration of graininess and ink savings.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to perform colorseparation in consideration of image graininess caused by spot color inkdots when use of chromatic primary color inks and spot color inks ispossible.

The invention in a first aspect thereof provides a color separationmethod for determining ink quantity of each ink, in order to reproduceany color using a plurality of color inks on a printing medium. Thismethod comprises the steps of (a) defining an ink set that includes asuseable inks a plurality of chromatic primary color inks that, when usedin combination, can reproduce achromatic color, and at least one spotcolor ink of hue different from any of the plurality of chromaticprimary color inks; (b) where color reproduced on the print mediumaccording to any one input color is termed “reproduction color”, thecombination of ink quantities in the ink set for reproducing thereproduction color on the print medium is termed the “color separationink quantity set”, and the color space representing ink quantities ofthe plurality of chromatic primary color inks as a base vector is termedthe “chromatic primary color space”, determining a plurality ofreproduction colors to be reproduced on the print medium according to aplurality of input colors in the chromatic primary color space; and (c)determining a plurality of color separation ink quantity sets forreproducing the plurality of reproduction colors. The step (c) includesexecuting, for each reproduction color, the steps of (c1) calculating alightness parameter correlated to lightness of the reproduction color;and (c2) adjusting the ink quantity of the spot color ink included inthe color separation ink quantity set in accordance with the lightnessparameter so that ink quantity decreases at a rate of change greaterthan the rate of change of the lightness parameter in the direction ofincreasing lightness.

According to this method, ink quantity of the spot color ink is adjustedso as to be reduced at a rate of change greater than the rate of changeof the lightness parameter, whereby standing out of the spot color inkin lighter areas can be reduced, and graininess in the image improved.

The invention in a second aspect thereof provides a color separationmethod for determining ink quantity of each ink, in order to reproduceany color using a plurality of color inks on a printing medium. Thismethod comprises the steps of: (a) defining an ink set that includes asuseable inks a plurality of chromatic primary color inks that, when usedin combination, can reproduce achromatic color, and at least one spotcolor ink of hue different from any of the plurality of chromaticprimary color inks; (b) where color reproduced on the print mediumaccording to any one input color is termed “reproduction color”, thecombination of ink quantities in the ink set for reproducing thereproduction color on the print medium is termed the “color separationink quantity set”, and the color space representing ink quantities ofthe plurality of chromatic primary color inks as a base vector is termedthe “chromatic primary color space”, determining a plurality ofreproduction colors to be reproduced on the print medium according to aplurality of input colors in the chromatic primary color space; and (c)determining a plurality of color separation ink quantity sets forreproducing the plurality of reproduction colors. The proportion ofactual ink quantity of the spot color ink to the maximum ink quantity ofthe spot color ink determined according to the reproduction color istermed “spot color ink usage rate.” When the hue of the reproductioncolor is reproduced by means of a combination of one chromatic primarycolor ink and one spot color ink, the one chromatic primary color ink istermed the “primary color component ink”, and the one spot color ink istermed the “spot color component ink.” The step (c) includes executing,for each reproduction color, the steps of: (c1) calculating a lightnessparameter value correlated to lightness of the reproduction color; and(c2) providing first and second reproduction colors, the first andsecond reproduction colors having a lightness parameter value within afirst high lightness range including a brightest portion of thelightness parameter value, the first and second reproduction colorshaving a same primary color component ink, a same spot color componentink, and a same lightness parameter value, the first reproduction colorhaving hue relatively close to that of the primary color component ink,the second reproduction color having hue relatively close to that of thespot color component ink; and determining ink quantity of the spot colorink included in the color separation ink quantity set so that a usagerate of the spot color ink for the second reproduction color is smallerthan a usage rate of the spot color ink for the first reproductioncolor.

According to this method, in bright image areas, the ink quantity ofspot color ink is determined in such a way that the usage rate of spotcolor ink in areas of hue relatively close to the spot color ink issmaller than the usage rate of spot color ink in areas of hue relativelyclose to the chromatic primary color ink. Accordingly, in areas of huerelatively close to the spot color ink, the usage rate of spot color inkis lower, whereby standing out of the spot color ink in lighter areascan be reduced, and graininess in the image improved. In areas of huerelatively close to the chromatic primary color ink, the usage rate ofspot color ink is higher, and ink quantities of the plurality ofchromatic primary color inks are smaller, providing savings in theamounts of ink used.

The present invention may take any of a number of different embodiments,for example, an image data conversion method and device, printing methodand printing device, or color conversion lookup table creation methodand device employing the color separation method; a computer program forrealizing the functions of such methods or devices; a storage mediumhaving such a computer program recorded thereon; a data signalcontaining such a computer program and embodied in a carrier wave, orthe like.

These another other objects, features, aspects, and advantages of theinvention will be understood from the following description of thepreferred embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a printing system arrangement.

FIG. 2 is a simplified schematic of printer 20.

FIG. 3 is a block diagram of printer 20 arrangement.

FIG. 4 illustrates ink nozzle arrangement on the lower face of printhead 28.

FIGS. 5( a)-5(c) depict ink sets.

FIG. 6 is a flowchart depicting the color reproduction processingroutine.

FIG. 7 depicts color patches.

FIG. 8 is a flowchart depicting a color separation process processingroutine.

FIG. 9 is a flowchart depicting the processing routine for calculating afinal color separation ink quantity set.

FIGS. 10( a), 10(b) show temporary ink quantities.

FIGS. 11( a), 11(b) are simplified illustrations of calculating finalcolor separation ink quantity of spot color ink.

FIG. 12 is a flowchart depicting a color separation process processingroutine.

FIGS. 13( a)-13(c) depict a primary color space.

FIGS. 14( a), 14(b) are simplified illustrations of calculation ofextended chromatic color.

FIG. 15 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set.

FIGS. 16( a), 16(b) show temporary ink quantities.

FIGS. 17( a)-17(d) are simplified illustrations of calculating finalcolor separation ink quantities of spot color inks.

FIG. 18 is a graph of the relationship between lightness and theproportion of temporary ink quantity versus maximum ink quantity.

FIG. 19 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set.

FIG. 20 is a graph of the relationship between lightness and theproportion of limit value to the maximum value assumable by the spotcolor ink quantity parameter.

FIGS. 21( a)-21(c) are simplified illustrations of calculating finalcolor separation ink quantities of spot color inks.

FIG. 22 is a flowchart depicting a color separation process processingroutine.

FIG. 23 is a flowchart depicting a color separation process processingroutine.

FIG. 24 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set.

FIGS. 25( a)-25(e) illustrate relationships between spot color ink usagerate and ink quantities of other inks.

FIGS. 26( a), 26(b) show temporary ink quantities.

FIG. 27 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set.

FIGS. 28( a), 28(b) show temporary ink quantities.

FIG. 29 is a graph of the relationship between lightness and theproportion of temporary ink quantity versus maximum ink quantity.

FIGS. 30( a)-30(c) illustrate another example of ink components.

FIGS. 31( a)-31(c) illustrate another example of ink components.

FIGS. 32( a)-32(c) illustrate another example of ink components.

FIGS. 33( a)-33(c) illustrate another example of ink components.

FIGS. 34( a)-34(c) illustrate another example of ink components.

FIGS. 35( a)-35(c) illustrate another example of ink components.

FIG. 36 illustrates another example of ink components.

FIG. 37 illustrates another example of ink components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the invention are described hereinbelow, in thefollowing order.

A. Device Arrangement B. Method for Creating Color Conversion LookupTable C. Embodiment 1

C1. Example 1 of Color Separation Process in Embodiment 1

C2. Example 2 of Color Separation Process in Embodiment 1

C3. Examples 1-3 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 1

C4. Example 3 of Color Separation Process in Embodiment 1

D. Embodiment 2

D1. Example 1 of Color Separation Process in Embodiment 2

D2. Examples 1-3 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 2

D3. Other Example of Color Separation Process in Embodiment 2

E. Ink Set Variations F. Variations A. Device Arrangement

FIG. 1 is a block diagram showing the arrangement of a printing systemin a first embodiment of the invention. This printing system comprises acomputer 90 as a image data processing device, and a color printer 20 asa printer portion. Computer 90 and color printer 20 can together betermed a “printing device”, in a broader sense.

On computer 90, an application program 95 runs on a predeterminedoperating system. This operating system includes inter alia a videodriver 91, and a printer driver 96; the application program 95 sendsprint data PD to printer 20 via these drivers. Application program 95,which performs functions such as image retouching, also performs desiredprocessing of images to be processed, as well as displaying images on aCRT 21 via the video driver 91.

When the application program 95 issues a print command, the printerdriver 96 of computer 90 receives image data from the applicationprogram 95, and converts this into print data PD to be supplied to theprinter 20. In the example shown in FIG. 1, printer driver 96 includes aresolution conversion module 97, a color conversion module 98, ahalftone module 99, a rasterizer 100, and a color conversion lookuptable LUT.

Resolution conversion module 97 has the function of converting theresolution (i.e. the number of pixels per unit of distance) of colorimage data produced by the application program 95 into print resolution.The resolution-converted image data is information for an image stillcomposed of three color components RGB. Color conversion module 98,while making reference to the color conversion lookup table LUT,converts the RGB image data (input color image data) for each pixel intomultitone data (second color image data) of a plurality of ink colorsutilizable by the printer 20.

The color-converted multitone data has gray level value for 256 tones,for example. Halftone module 99 executes a so-called “halftone” processto generate halftone image data. The halftone image data is arranged byrasterizer 100 in the order in which the data will be sent to theprinter 20, for output as final print data PD. Print data PD includesraster data that indicates dot recording status during each main scan,and data that indicates sub scan feed distance.

The printer driver 96 corresponds to a program for performing thefunction of generating print data PD. The program for performing thefunction of printer driver 96 is provided recorded in computer-readableform on a recording medium. Typical recording media include flexibledisks; CD-ROM; magnetooptical disks; IC cards; ROM cartridges; punchcards; printed matter imprinted with symbols such as bar codes; computerinternal storage devices (e.g. RAM, ROM or other type of memory) andexternal storage devices; and other such computer-readable storagemedia.

FIG. 2 is a simplified schematic of printer 20. Printer 20 comprises asub-scan feed mechanism for advancing printing paper PP in thesub-scanning direction by means of a paper feed motor 22; a main scanfeed mechanism for reciprocating a carriage 30 in the axial direction ofa platen 26 (i.e. main scanning direction) by means of a carriage motor24; a head drive mechanism for driving a print head unit 60 carried oncarriage 30, and controlling ink ejection and dot formation thereby; anda control circuit 40 for exchange signals with the paper feed motor 22,carriage motor 24, print head unit 60 and a control panel 32. Thecontrol circuit 40 is connected to computer 90 via a connector 56.

The sub-scan feed mechanism for advancing print paper PP in thesub-scanning direction comprises a gear train (not shown) fortransmitting rotation of the paper feed motor 22 to the platen 26 andpaper feed rollers (not shown). The main scan feed mechanism forreciprocating the carriage 30 comprises a slide rail 34 extendingparallel to the axis of platen 26, for slidably retaining the carriage30; a pulley 38 around which is passed an endless belt 36 that extendsto the carriage motor 24; and a position sensor 39 for sensing the homeposition of the carriage 30.

FIG. 3 is a block diagram of the printer 20 arrangement, focusing on thecontrol circuit 40. The control circuit 40 constitutes anarithmetic/logic circuit comprising a CPU 41, programmable ROM (PROM)43, RAM 44, and a character generator (CG) for storing character dotmatrices. The control circuit 40 additionally comprises a dedicated I/Fcircuit 50 dedicated exclusively to interface with external motors,etc.; a head drive circuit 52 connected to the dedicated I/F circuit 50,for driving the print head unit 60 to eject ink; and a motor drivecircuit 54 for actuating the paper feed motor 22 and carriage motor 24.The dedicated I/F circuit 50 includes a parallel interface circuitallowing it to receive print data PD supplied by computer 90 viaconnector 56. Circuitry within dedicated I/F circuit 50 is not limitedto a parallel interface circuit; in view of ease of connection tocomputer 90, communication speed, etc., a universal serial bus interfacecircuit or the like could be used instead. Printer 20 executes printingin accordance with the print data PD. RAM 44 functions as a buffermemory for temporarily storing raster data.

Print head unit 60 has a print head 28, and accommodates installation ofink cartridges containing compatible inks. Print head unit 60 detachablyinstalls onto printer 20 as a single unit. That is, to replace printhead 28, the entire print head unit is replaced.

FIG. 4 illustrates ink nozzle arrangement on the lower face of printhead 28. On the lower face of print head 28 are arranged a nozzle groupfor ejecting cyan ink C, a nozzle group for ejecting magenta ink M, anozzle group for ejecting black ink K, a nozzle group for ejecting redink R, a nozzle group for ejecting violet ink V, and a nozzle group forejecting yellow ink Y. In this embodiment, it is possible to use an inkset composed of the six inks C, M, Y, R, V, and K. In the example ofFIG. 4, the plurality of nozzles Nz of a single nozzle group arearranged in a line in the sub-scanning direction SS, but could insteadbe arranged in a zigzagged configuration.

FIG. 5( a) depicts ink components of the C, My, Y, R, V and K color inksof the ink set. Each color ink is ion exchange water-based, containingvarious kinds of colorant consisting of dyes or pigments to impart thedesired color, or a mixed solution additionally containing a suitableamount of ethylene glycol or the like added to modify viscosity. Thetype of colorant is indicated by the color index (CI) of the colorant.

Cyan ink C, magenta ink M, and yellow ink Y may be used in combinationto reproduce gray (achromatic color), and correspond to chromaticprimary color inks. Red ink R and violet ink V are different in hue fromany of the chromatic primary color inks (CMY), and correspond to spotcolor inks. The red ink R has a hue between those of the yellow ink Yand magenta ink M; the violet ink V has a hue between those of themagenta ink M and cyan ink C.

Mixed colors of the chromatic primary color inks C, M, Y can reproducehue and saturation substantially identical to the colors of the spotcolor inks R, V. Here, each ink quantity of chromatic primary color inkin a color mixture, relative to the ink quantity of a spot color ink,i.e., each ink quantity of chromatic primary color ink when the inkquantity of a spot color ink is designated as 1, is termed substitutionink quantity. Inks of each of the colors C, M, Y and inks of each of thecolors R, V can reproduce substantially the same colors when substitutedon the basis of substitution ink quantities.

FIGS. 5( b) and 5(c) each show an experimental result of measuringsubstitution ink quantities utilizing the ink set shown in FIG. 5( a).This experimental result was obtained by colorimetric measurement andcomparison of a color patch created with a color mixture of thechromatic primary color inks C, M, Y with color patches of the spotcolor inks R, V respectively. FIG. 5( b) gives substitution inkquantities for red ink R, with substitution ink quantities of the colorsCMY being denoted by the symbols wCR, wMR, wYR. FIG. 5( c) givessubstitution ink quantities for violet ink V, with substitution inkquantities of the colors CMY being denoted by the symbols wCV, wMV, wYV.In the right column of each table is given the total value ofsubstitution ink quantities.

In this way, substitution ink quantities for each of the spot color inksR, V are values in which two of the three ink quantities are greaterthan zero, with the one remaining ink quantity being zero. That is, eachspot color ink R, V can be separated into two chromatic primary colorcomponents. In the ink set given in FIGS. 5( a)-5(c), a color mixture ofchromatic primary color inks can be substituted by a spot color ink, inan amount smaller than the total value for the ink quantities. As aresult, by actively employing the spot color inks, it becomes possibleto reproduce substantially identical hue and saturation with smallerquantities of ink. By reducing ink quantity it is also possible toreproduce higher lightness. Additionally, by employing a spot color inkin an ink quantity substantially equal to that of a chromatic primarycolor ink color mixture, it is possible to reproduce higher saturation.Thus, even where a limit is imposed on the total value of ink quantityused (i.e., an ink duty limit, described in detail later), by using aspot color ink, it is nevertheless possible to reproduce highersaturation than would be possible when reproduced with a chromaticprimary color ink color mixture. By utilizing chromatic primary colorink and spot color ink in this way, it becomes possible to reproduce awider range of color than would be reproducible with chromatic primarycolor inks only.

The two spot color inks R, V also have hues different from one another.Additionally, these inks R, V have different principal component primarycolor inks from one another; the principal component primary color inksdenote the two inks that have the largest values of the substitution inkquantities among the color inks CMY. In the example of FIGS. 5( b) and5(c), the principal component primary color inks of the red ink R aremagenta ink M and yellow ink Y. The principal component primary colorinks of the violet ink V are cyan ink C and magenta ink M. In thisexample, the difference in the principal component primary color inks isthe yellow ink Y and cyan ink C. As a result, the two spot color inks R,V can extend their color reproduction range in areas of mutuallydifferent hue. Thus, a wider range of color can be reproduced, ascompared to the case where spot color inks of mutually similar hue areused.

In the ink set shown in FIG. 5( a), the spot color inks R, V containdifferent colorant than the chromatic primary color inks C, M, Y. Thus,by using the spot color inks in place of color mixtures of the chromaticprimary color inks C, M, Y, reproduction of hues similar to those of thespot color inks can be improved.

Printer 20 having the hardware configuration described above advancesthe printer paper PP by means of the paper feed motor 22 whilereciprocating the carriage 30 by means of the carriage motor 24, whileat the same time actuating the piezo elements of print head 28 to ejectdrops of ink of each color, to produce ink dots and form a multicolor,multitone image on the printer paper PP.

B. Method for Creating Color Conversion Lookup Table

FIG. 6 is a flowchart depicting the color reproduction processingroutine in the first embodiment of the invention. In Steps S10-S70, acolor conversion lookup table LUT (FIG. 1) is created for the purpose ofcarrying out color reproduction.

First, in Step S10, one combination of a print medium and an ink set foruse in printing is selected. It is assumed that for typical printing,one print medium selected by the user from among several types of printmedia (plain paper, gloss paper, matte paper etc.). In certain types ofprinter, the ink set to be used can be selected from among a number oftypes of ink set (for example, a dye ink set and a pigment ink set).Color reproduction of printed matter depends on the print medium and theink set. Accordingly, in this embodiment, the processes of Steps S10-S60are executed for each combination of print medium and ink set, and acolor conversion lookup table LUT appropriate for each combination iscreated. The type of print medium and type of ink set assumed to be usedin printer 20 are typically displayed on a screen (not shown) forsetting the print parameters of the printer driver 96.

In Step S20, a color separation process to convert a primary color tonevalue set expressed in a primary color system into a second tone valueset expressed in a reproduction color system is executed. The primarycolor system is a color system represented by color ink quantities ofthe chromatic primary color inks CMY; the reproduction color system is acolor system represented by ink quantities of the color inks used duringprinting. This primary color tone value set is composed of inkquantities of the each of the plurality of chromatic primary color inksC, M, Y. Ink quantities of the chromatic primary color inks C, M, Y arevalues representing a range from the minimum possible value (zero) tothe maximum value (ink quantity reproducing a completely solid area) in256 levels from 0 to 255, for example. In this embodiment, a solid areais reproduced by ejecting ink onto all pixels. Thus, ink quantity whenreproducing such a solid area may be assigned a value of 100%.

In this Step S20, first, a plurality of primary color tone value setsare prepared. Ink quantities of each of the chromatic primary color inksC, M, Y in these primary color tone value sets are preferablydistributed over the entire possible range (0%-100%), and in especiallypreferred practice will be distributed uniformly throughout the entirerange. As a plurality of value for ink quantity, it would be possible touse the eleven values of 0, 25, 50, 75, 100, 125, 150, 175, 200, 225,and 255, for example. Apparent change in reproduction color versus thechange in tone values of ink quantities may differ depending on ink tonevalue in some instances. In such instances, it is preferable to prepareink quantities of inks using smaller intervals in the range of tonevalues that have greater apparent change in color. By so doing, it ispossible to create a color conversion lookup table LUT corresponding tofiner changes in apparent color change.

Next, the plurality of primary color tone value sets are converted tosecond tone value sets expressed in the reproduction color system. Thereproduction color system is a color system represented by inkquantities of the ink set employed at the time of printing, for example,color ink quantities for the chromatic primary color inks CMY and thespot color inks RV. A second tone value set consists of valuesrepresenting a range of minimum (0%) to the maximum (100%) possible forCMYRV color ink quantities, in 256 levels from 0 to 255, for example. Adetailed description of the color separation process from the primarycolor system to the reproduction color system will be made later.

In Step S30, a plurality of kinds of color patches corresponding to theplurality of primary color tone value sets are created. FIG. 7 depictsexemplary color patches created in the embodiment. The vertical axis isthe tone value of magenta ink M in the primary color tone value setsprepared in Step S20 above; the horizontal axis is yellow ink Y tonevalue. Each color patch is reproduced by ink quantities derived byconversion of a tone value set according to the color separation processof Step S20. The example of FIG. 7 shows a case in which the tone valuefor cyan ink C in the primary color tone value sets has been set tozero. In actual practice, a plurality of kinds of color patchescorresponding to a plurality of tone values for cyan ink C would becreated, but are not shown in the drawing. In this way, in Step S30, aplurality of kinds of color patches corresponding to the plurality ofprimary color tone value sets prepared in Step S20 are created.

In Step S40 (FIG. 6), using a calorimeter, the plurality of colorpatches created in Step S30 are subjected to calorimetric measurement.The data obtained as a result of colorimetric measurement is expressedin a color system that is independent of any device (i.e., a printer,monitor, etc.), for example, the L*a*b color system or XYZ color system.In this way, by subjecting each color patch to colorimetric measurementin Step S40, it is possible to determine “primarycolor/device-independent color system correspondence relationships” forprimary color systems and device-independent color systems. Also, as aresult of colorimetric measurement it is possible to verify a colorgamut reproducible by printer 20, in a device-independent color system.

In Step S50, a correspondence relationship between an arbitrary firstcolor system and a primary color system is established on the basis ofthe “primary color/device-independent color system correspondencerelationships” derived in Step S40. The first color system is the colorsystem of input color image data of the color conversion lookup tableLUT; the sRGB color system could be used, for example. “First colorsystem/device-independent color system correspondence relationships” forsuch first color systems and device-independent color systems have beenpre-established. Thus, employing the “first colorsystem/device-independent color system correspondence relationships” andthe “primary color/device-independent color system correspondencerelationships” derived in Step S40, it is possible to establish acorrespondence relationship between a first color system and a primarycolor system. The color reproduction range in the first color system andthe color reproduction range of the printer may in some instances havenon-overlapping portions. In such cases, preferred practice is toeffectively utilize the entire gamut of each by establishingappropriately expanded or reduced correspondence relationships.

Once a first correspondence relationship between a first color systemand primary color system (Step S50) and a second correspondencerelationship between a primary color system and a reproduction colorsystem (Step S20) have been established in this way, in Step S60, acolor conversion lookup table LUT (FIG. 1) for reproducing theestablished correspondence relationships is created. The colorconversion lookup table LUT in this embodiment has RGB image data asinput, and outputs multitone image data for the six ink colors shown inFIG. 4. Accordingly, when creating the color conversion lookup tableLUT, first, a primary color tone value set, represented by CMY dependingon tone values of the RGB image data, is calculated. Next, a second tonevalue set, i.e., ink quantity for each ink, depending on this first tonevalue set is determined by means of a color separation process,described later. Correspondence relationships having RGB image datavalues as input and ink quantity of each ink as output are then storedin the color conversion lookup table LUT.

In Step S70 in FIG. 6, a decision is made as to whether the processes ofSteps S10-S60 have been completed for all combinations of ink set andprint medium assumed to be used by printer 20. In the event that not allprocessing has been completed, the processes of Steps S10-S60 arerepeated; if completed, the system moves to the next Step S80.

In Step S80, the plurality of created color conversion lookup tables LUTare incorporated into printer driver 96 (FIG. 1). Printer driver 96 is acomputer program for realizing on computer 90 the function of creatingprint data PD to be sent to printer 20. Color conversion lookup tablesLUT are installed together with printer driver 96 on computer 90, asdata to which printer driver 96 refers. A printer driver 96incorporating color conversion lookup tables LUT is typically providedby the vendor of the printer 20.

In Step S90 in FIG. 6, the user executes printing using printer 20. Atthis time, a lookup table suitable for the particular combination ofprint medium and ink set used for actual printing is selected from amongthe color conversion lookup tables LUT for all combinations of printmedia and ink sets, and printing is executed. The combination of printmedium and ink set used for actual printing is selected by the use froma screen (not shown) for setting print parameters of printer driver 96.

C. Embodiment 1 C1: Example 1 of Color Separation Process in Embodiment1

FIG. 8 is a flowchart depicting the color separation process processingroutine in Example 1. In this color separation process, a process toconvert from a primary color system to a reproduction color system isexecuted. In Step S500 in FIG. 8, an ink set composed of the chromaticprimary color inks C, M, Y and the spot color inks R, V as useable inksis established.

In Step S510, a provisional color separation ink quantity set I(Cp, Mp,Yp, Rp, Vp) for reproducing on a print medium a reproduction colorcorresponding to an input color I(Ci, Mi, Yi) is calculated. In Example1, it is assumed that input color I(Ci, Mi, Yi) matches a reproductioncolor to be reproduced. As there are innumerable color separation inkquantity sets that can give an arbitrary reproduction color, theprovisional color separation ink quantity set is determined byestablishing a certain specific condition. For example, in this Example,the provisional color separation ink quantity set P(Cp, Mp, Yp, Rp, Vp)is determined so as to minimize total ink quantity.

As depicted in FIGS. 5( a)-5(c), a spot color ink color component may beseparated into two chromatic primary color ink color components.Accordingly, total ink quantity is smaller the greater the ink quantityof spot color ink. The provisional color separation ink quantity set Pof this Example is uniquely selected because it is that which, of allprovisional color separation ink sets able to reproduce an input color Iwith a given reproduction color, has the smallest total ink quantity. Aswill be understood from other examples described later, the provisionalcolor separation ink quantity set P may also be determined according toother criteria.

In Step S520, a final color separation ink quantity set O(Co, Mo, Yo,Ro, Vo) is determined on the basis of this provisional color separationink quantity set P(Cp, Mp, Yp, Rp, Vp) (described in more detail later).In Step S530, a decision is made as to whether the processes of StepsS510, 520 have been completed for all reproduction colors needed tocreate a lookup table; the processes of Steps S510, 520 are repeateduntil processing for all reproduction colors has been completed.

FIG. 9 is a flowchart depicting in detail the procedure of Step S520. InStep S600, two temporary ink quantities Rtmp, Vtmp for the two spotcolor inks R, V from provisional color separation ink quantity set P aredetermined. FIG. 10( a) is a graph for calculating a temporary inkquantity Rtmp from the provisional color separation ink quantity Rp ofthe red ink component R. Over a first range R1 in which provisionalcolor separation ink quantity Rp goes from zero to a first value Rstart,temporary ink quantity Rtmp is set to zero. Over a second range R2 inwhich provisional color separation ink quantity Rp goes from the firstvalue Rstart to a second value Rend, temporary ink quantity Rtmp risesfrom zero in linear fashion. However, over this second range R2, thetemporary ink quantity Rtmp is maintained at a value smaller than theprovisional color separation ink quantity Rp. Over a third range R3 inwhich provisional color separation ink quantity Rp goes above the secondvalue Rend, temporary ink quantity Rtmp is set to a value equal to theprovisional color separation ink quantity Rp.

FIG. 10( b) is a graph for calculating the proportion k of temporary inkquantity Rtmp to provisional color separation ink quantity Rp. As willbe understood from the graph, over the first range R1 proportion k iszero, with proportion k increasing monotonically from zero to 1 over thesecond range R2. Over the third range R3, proportion k is constant at 1.

The reason for setting temporary ink quantity Rtmp in the manner ofFIGS. 10( a) and 10(b) is as follows. When provisional color separationink quantity Rp is small, in most cases the image reproduced by thereproduction colors thereof is an image of a so-called highlight areas(high lightness areas). A feature of the spot color inks R, V is thattheir saturation and density are higher than those of the chromaticprimary color inks. Since dots of spot color inks have higher saturationand density, they will tend to stand out in highlight areas, and maypossibly worsen graininess in the image. Accordingly, the use of smallerink quantity of spot color ink in highlight areas is preferable. Also,since reducing the ink quantity of spot color ink increases the inkquantity of chromatic primary color ink, the total number of ink dotsincreases. In this aspect as well, reducing ink quantity of spot colorink in highlight areas can improve graininess in an image. Accordingly,by setting ink quantity Rtmp of spot color ink to zero over the firstrange R1 of FIG. 10( a), formation of dots of spot color ink inhighlight areas can be avoided. By so doing, it is possible tosignificantly improve image graininess in highlight areas.

On the other hand, over the third range R3 in which provisional colorseparation ink quantity Rp is large, it is preferable for ink quantityof spot color ink to be larger so as to give high image saturation.Accordingly, over this range R3, provisional color separation inkquantity Rp per se is set as the temporary ink quantity Rtmp. In thesecond range R2, temporary ink quantity Rtmp increases in linearfashion, so that over the three ranges R1-R3, temporary ink quantityRtmp changes smoothly, rather than changing in stepwise fashion.Alternatively, temporary ink quantity Rtmp may change along a curve,instead of changing in linear fashion. Typically, it is preferable fortemporary ink quantity Rtmp to change continuously at a constant rate ofincrease.

Since the red ink provisional color separation ink quantity Rp relatingto a given reproduction color represents the density of the red inkcomponent of that reproduction color, this ink quantity Ro can bethought of as an index indicating lightness in relation to the red inkcomponent. When red ink provisional color separation ink quantity Rprelating to a given reproduction color is large, lightness of thereproduction color tends to be low. Accordingly, the provisional colorseparation ink quantity Rp can be thought of as a lightness parametervalue correlated to lightness of the reproduction color.

As regards violet ink V as well, a temporary ink quantity Vtmp can bedetermined from the provisional color separation ink quantity Vp, by thesame method as for red ink R.

In Step S610 in FIG. 9, final color separation ink quantities Ro, Vo aredetermined from temporary ink quantities Rtmp, Vtmp. The final colorseparation ink quantities Ro, Vo are determined by correcting thetemporary ink quantities Rtmp, Vtmp as needed in order to meet ink dutylimits. Herein, ink duty limit refers to a limit relating to inkquantity useable per unit of area of a print medium. Ink duty limitsinclude ink-by-ink limits, limits as to total ink quantity for two kindsof ink, limits as to total ink quantity for all inks, or the like.

When executing Step S610, first, temporary ink quantities for thechromatic primary color inks C, M, Y, which in conjunction with thetemporary ink quantity pair Rtmp, Vtmp are needed to reproduce desiredreproduction color, are calculated. A decision is then made as towhether the temporary ink quantity set meets ink duty limits.

FIG. 11( a) depicts an exemplary permissible ink duty limit range RAdrawn on a two-dimensional color space defined by two spot color inks R,V. In the drawing are also shown an exemplary coordinate point P(Rp, Vp)of a provisional color separation ink quantity pair and coordinate pointPrv(Rtmp, Vtmp) of a temporary ink quantity pair. Outside edges of thepermissible area RA are defined not only by boundary lines determined byindependent duty limits for the spot color inks, but also by boundarylines determined by duty limits for other inks. For example, boundaryline LRVM1 at the lower left of permissible area RA corresponds to theink duty limit for magenta ink. As described in FIGS. 5( a)-5(c), thetwo spot color inks each include a magenta component, and thus if inkquantities of the spot color inks are reduced, the ink quantity ofmagenta ink M will increase. Accordingly, in order to meet the dutylimit for magenta ink M, ink quantities of the spot color inks must beto the right of and above the boundary line LRVM1. As will be understoodfrom the preceding description, the issue of whether ink duty limits aremet takes into consideration ink quantities of all inks that make up anink set. For convenience, in the description hereinbelow, it will beassumed that all ink duty limits are met when the temporary ink quantitypair Rtmp, Vtmp lie within permissible area RA.

Where the color coordinate point Prv of a temporary ink quantity pairlies within permissible area RA as shown in FIG. 11( a), the temporaryink quantity pair Rtmp, Vtmp per se is adopted as the final colorseparation ink quantity pair Ro, Vo. On the other hand, where thecoordinate point Prv of a temporary ink quantity pair color fallsoutside permissible area RA as shown in FIG. 11( b), the final colorseparation ink quantity pair Ro, Vo is determined by means of correctingthe temporary ink quantity pair Rtmp, Vtmp so as to meet ink dutylimits. At this time, a point located within the permissible area RA ofthe two-dimensional color space, and situated in proximity to the colorcoordinate point Prv of the temporary ink quantity pair, is selected asthe color coordinate point of the final color separation ink quantitypair Ro, Vo. In the example depicted in FIG. 11( b), there are drawnthree such points Oa, Ob, Oc that could be selected as such a colorcoordinate point of the final color separation ink quantity pair. Thefirst point Oa(Ro, Vo) is a point that equalizes the ratio of thetemporary ink quantity pair Rtmp:Vtmp with the ratio of the final colorseparation ink quantity pair Ro:Vo. The second point Ob is a point atwhich a straight line connecting the color coordinate point P of theprovisional color separation ink quantity pair (Rp, Vp) with the colorcoordinate point Prv of the temporary ink quantity pair intersects aboundary line of the permissible area RA. The third point Oc is thepoint closest to the color coordinate point Prv of the temporary inkquantity pair within the permissible area RA.

While any of these three points Oa, Ob, Oc could be selected as thecolor coordinate point of the final color separation ink quantity pair,selecting the first point Oa is especially preferred. The reason is thatsince the temporary ink quantity pair Rtmp, Vtmp is determined accordingto the relationships of FIGS. 10( a) and 10(b) by way of preferred inkquantities for improving graininess in an image, if the ratio thereof.Rtmp:Vtmp is maintained, it is possible to achieve a good balancewhereby dots of neither of the two spot color inks will stand out, withresultant high probability of being able to improve graininess in theimage.

Further, since ink duty limits function as limits only when the quantityof ink producing an image is considerable, they are a problem mainly inimage areas of low lightness (i.e., image areas of high density).Accordingly, in highlight areas having high lightness, ink duty limitsare not a problem, and temporary ink quantities Rtmp, Vtmp establishedon the basis of the relationships of FIGS. 10( a) and 10(b) are adoptedper se as the final color separation ink quantities Ro, Vo. Accordingly,since the number of ink dots of the spot color inks R, V in highlightareas is fairly small, it is possible to improve graininess in theimage.

In Step S620 in FIG. 9 are determined ink quantities Co, Mo, Yo for theother inks which, in conjunction with the final color separation inkquantity pair Ro, Vo, are needed to reproduce desired reproductioncolor. As a result, a final color separation ink quantity set (Co, Mo,Yo, Ro, Vo) for reproducing desired reproduction color is determined.

In this way, in Example 1, for each of the spot color inks R, V, finalcolor separation ink quantity Ro, Vo is smaller in association withsmaller lightness parameter values (provisional color separation inkquantities) Rp, Vp thereof, whereby numbers of ink dots of the spotcolor inks is reduced in highlight areas, with a corresponding increasein the number of ink dots of the chromatic primary color inks. Aresultant advantage is that image graininess in highlight areas isimproved. In particular, since over a first range R1 (FIG. 10( a)) inwhich provisional color separation ink quantities Rp, Vp assumerelatively small values, temporary ink quantities Rtmp, Vtmp are set tozero, in highlight areas final color separation ink quantities Ro, Vowill also be set to zero, significantly improving image graininess.However, it would also be possible to set temporary ink quantities Rtmp,Vtmp to relatively small, non-zero values over this first range R1.

C2. Example 2 of Color Separation Process in Embodiment 1

FIG. 12 is a flowchart depicting a processing routine in a secondExample. In Example 1 described previously, primary color system inputvalues per se are employed as the reproduction colors (the colors of thereproduction color system); in Example 2, however, colors of highersaturation than input colors are employed as the reproduction colors.The reason this is possible lies in the fact that the spot color inks R,V included in the reproduction color system can achieve higher levels ofsaturation as regards to red color and violet color than color mixturesof chromatic primary color inks.

In Step S100 in FIG. 12, an ink set composed of the chromatic primarycolor inks C, M, Y and the spot color inks R, V as useable inks isestablished.

Next, in Step S110, an ink duty limit, which limits ink quantity, isestablished for each color of the ink set. Ink duty limit is establishedwith reference to type of ink and print medium (described in detaillater). Input colors in the primary color space are represented usingtone values in the possible range (0%-100%) for CMY color inkquantities, in 256 levels from 0 to 255. The color separation inkquantity set in the reproduction color system is represented using tonevalues in the possible range (0%-100%) for CMYRV color ink quantities,in 256 levels from 0 to 255.

FIGS. 13( a), (b) depict a primary color space having CMY color inkquantities as base vectors. Colors expressed in the primary color spaceare depicted as points of a cube represented by CMY tone values of 0-255in the primary color space. The cube is the area of values that can beassumed by ink quantities of the CMY colors of the chromatic primarycolor inks. Hereinafter, this cube will be referred to as a “colorsolid”, and, of the six faces of the color solid, the three faces facingthe origin W (the three faces surrounding K(C=M=Y=100%)) will be termedtype 1 outer shell planes. In other words, type 1 outer shell planes arecomposed of points of colors having at least one chromatic primary colorink quantity that is at 100%, and having at least one chromatic primarycolor ink quantity that is at less than 100%. Where the line connectingthe origin W with point K is termed a “achromatic color line”, thedistance between any particular point in the primary color space and theachromatic color line can be used as an index of saturation of theparticular point. Where a point projected on the achromatic color lingfrom the particular point is termed a “projection point”, the distancebetween the projection point and the origin W can be used as an index oflightness of the particular point. The direction from the projectionpoint to the particular point in the primary color space can be used asan indicator of hue of the particular point.

In FIGS. 13( a), 13(b), the type 1 outer shell plane having Y at maximum(Y=255) is depicted with hatching. A single color m is noted on thehatched type 1 outer shell plane. This color m is established as theoutermost shell chromatic color m in Step S120 of FIG. 12. In theexample of FIGS. 13( a), 13(b), outermost shell chromatic color m isestablished on the outer shell plane having the Y component at maximum,with tone values for the colors CMY designated, in CMY order, as Cm, Mm,and Ym (in this example, Ym=255).

In the color separation process of this Example, by sequentiallyexecuting the processes of Steps S130-S150 described later, it ispossible to derive a color separation ink quantity set associated withan input color I on a line connected the origin W with the outermostshell chromatic color m. Also, in this Example, in order to executecolor separation processing for a plurality of input colors I, aplurality of outermost shell chromatic colors are provided, and a seriesof processes (S130-S150) is executed on each outermost shell chromaticcolor.

In Step S130 in FIG. 12, there is also calculated an extended chromaticcolor em situated on the outer shell of the gamut reproducible using theCMYRV color inks of the ink set (FIG. 13( b)).

FIGS. 14( a) and 14(b) show calculation of extended chromatic color em.In the example of FIGS. 14( a) and 14(b), in order to simplify thedescription, it is assumed that two types of ink, namely, cyan ink C andmagenta ink M, are useable as chromatic primary color inks, and one typeof ink, namely violet ink V, is useable as spot color ink.

FIG. 14( a) depicts a primary color space. In this example, CMV colortone values can assume values in the range 0-100. Thus, an input colorexpressed in the primary color space is represented as a point inside asquare whose sides have length of 100. This square corresponds to thecolor solid described earlier. In the drawing, the type 1 outermostshell OL1 of the square is denoted by a thick line. This type 1outermost shell OL1 corresponds to the type 1 outermost shell describedearlier. On a line on which the cyan C of type 1 outermost shell OL1 isat maximum (C=100) there is established an outermost shell chromaticcolor m.

FIG. 14( b) depicts the range of possible values for hypothetical inkquantities for each color, when colors reproducible using a spot colorink V in addition to chromatic primary color inks CM are expressed in aprimary color system. Here, it is assumed that a 1:1 mixture of cyan inkC and magenta ink M can reproduce substantially identical hue andsaturation as an violet ink V in an equal amount. That is, for both cyanink C and magenta ink M, ink quantity substitutable for violet ink Vis 1. For example, color P1 in FIG. 14( b) is the color reproduciblewhen CM color tone values are each 100. It is also possible to reproducesubstantially the same color by substituting a V tone value for CM colortone values. For example, even if only the tone value of V is 100, i.e.,a V tone value is substituted for all CM color tone values, it isnevertheless possible to reproduce substantially the same color. Here,the tone value obtained by substituting tone values of chromatic primarycolor inks CM for all tone values of spot color ink V (in this example,C=100, M=100) can be used as a hypothetical tone value for representingin a primary color space colors reproducible using the colors CMV.

In this example, the following limitations are imposed with regard totone value of each ink.

(Condition 1-a) Tone value of each ink is 80 or less.(Condition 1-b) The sum of the tone values of the inks is 200 or less.

Limitations imposed on tone values by Conditions 1-a, 1-b can beexplained as follows. Basically, there is a limit as the quantity of inkthat can be absorbed by a print medium per unit of area. If a quantityof ink in excess of this limit is ejected, the unabsorbed excess ink mayproduce bleeding, or wrinkling of the print medium. Thus, it isdesirable to establish a limit for ink quantity used. Such an upperlimit for ink quantity, i.e. upper limit as to tone value, is termed theink duty limit. In some instances, appropriate values for ink duty limitwill differ depending on the type of ink. In such instances, picturequality of printed images can be improved by establishing a differentlimit value for each color. By establishing a limit value for the sum ofthe tone values of inks of each color (i.e. the sum of ink quantities),as does Condition 1-b, it is possible to avoid ejection of ink in aquantity exceeding the ink absorbing capacity of the print medium.Additionally, it is preferable to establish a limit value for the sum ofink quantities of any two types of ink, for areas that are reproduced bytwo-color mixtures. It is also is preferable to establish a limit valuefor the sum of quantities of many types of ink. By varying these limitvalues according to the type of print medium, it is possible to improvepicture quality of a printed image depending on the type of printmedium.

Such ink duty limits are represented in terms of tone values for thecolors of the useable inks CMV, but could instead be expressed in aprimary color space by using hypothetical tone values for each of thecolors CM derived using substitution ink quantities. In the example ofFIG. 14( b), since ink duty limit relationships among the CMV colors arerepresented linearly, they are represented as straight lines in theprimary color space. Thus, the area in which reproduction is possibleusing inks of the CMV colors within ranges that satisfy ink duty limitsis shown as an area enclosed by straight lines that correspond to theink duty limits. In FIG. 14( b), straight line LC is the straight linefor which C=80. The fact that it is sloped with respect to the C axis isbecause the use of violet ink V will allow the hypothetical tone valuesof the CM colors to have greater values. Thus, the area meeting thecondition C≦80 lies to the inside of this straight line LC. Straightline LCV is the straight line for which C+V=160. This straight linecorresponds to the limit C+V≦160, derived from the two limits C≦80 andV≦80. The area meeting the condition C+V≦160 lies to the inside of thisstraight line LCV.

The intersection point P2 of straight lines LC and LCV represents a Ctone value of 160 and a M tone value of 80. Since this color P2, interms of the tone value of C, does not fulfill ink duty limits(Condition 1-a), it cannot be reproduced when only inks of the twocolors CM are used. Here, of the tone values for the colors CM, thevalue of 80 is converted to a V tone value, whereupon tone values forthe colors CMV, i.e. color separation ink quantities, become 80, 0, 80,so that ink duty limits are met. That is, color P2 can be reproducedusing chromatic primary color inks CM and spot color ink V.

Also depicted in FIG. 14( b) are the following straight linescorresponding to ink duty limits. Straight line LCMV is the straightline for which C+M+V=200, straight line LMV is the straight line forwhich M+V=160, and straight line LM is the straight line for which M=80.As a result, color within area A enclosed by these straight lines is acolor that meets ink duty limits, and that is reproducible using spotcolor ink V. That is, where a hypothetical tone value derived bysubstituting chromatic primary color ink for spot color ink tone valuelies within area A, it can be reproduced using chromatic primary colorink and spot color ink.

Distances between the origin W and these straight lines LCV, LCMV, LMVare values that vary depending on substitution ink quantity of spotcolor ink. That is, the greater substitution ink quantity is, thegreater the distance between the origin W and the straight linecorresponding to each ink duty limit. As a result, greater substitutionink quantity means a larger area reproducible using chromatic primarycolor ink and spot color ink. Thus, from the standpoint of extendingreproducible area, a total value for substitution ink quantity ofgreater than 1 is preferred, with 1.5 or above being especiallypreferred. In the example of FIG. 14( b), the substitution ink quantityof violet ink V is 1 for each of the colors CM, so total substitutionink quantity is 2. In the example of the ink set of FIGS. 5( b) and5(c), red ink R substitution ink quantities, in the order CMY, are 0.0,0.71, and 2.86, for a total of 3.57. Violet ink V substitution inkquantities, in the order CMY, are 0.68, 2.89, and 0.0, for a total of3.57. Since each of the totals of substitution ink quantities of the twoinks RV is above 1.5, a wider color reproduction range can be obtainedthrough the use of these inks R, V. Where the total of substitution inkquantities of chromatic primary color inks is greater than 1, spot colorink, used in an ink quantity about equal to that of a chromatic primarycolor ink color mixture, makes it possible to reproduce high colorsaturation. By so doing, through the use of chromatic primary color inkand spot color ink, it is possible to reproduce a wider range of colorthan is reproducible using chromatic primary color inks alone.

The outer shell in an area meeting ink duty limits in this way isreferred to herein as a “reproduction color outer shell plane”. While areproduction color outer shell plane is represented by a reproductioncolor system for ink quantities of an ink set, it could be mapped into aprimary color system by substituting ink quantities of chromatic primarycolor ink for ink quantities of spot color inks according tosubstitution ink quantities. In the example of FIG. 14( b), an outershell line OL2 constituting the outer shell of area A corresponds to areproduction color outer shell plane when mapped to a primary colorsystem (hereinafter, outer shell line OL2 will be referred to asreproduction color outer shell line OL2). As regards the condition thatV≦80, since this condition is met anywhere within area A, acorresponding straight line diagram is not shown.

In FIG. 14( b), area A is shown hatched, and reproduction color outershell line OL2 is depicted as a thick line. An extended chromatic colorem is established on reproduction color outer shell line OL2. Extendedchromatic color em is a color located on the intersection point ofreproduction color outer shell line OL2 with a line segment passingthrough the origin W and the outermost shell chromatic color m. That is,the extended chromatic color em is a color represented by the longestextended chromatic color vector having the same direction as theoutermost shell chromatic color vector representing the outermost shellchromatic color m in the primary color space; furthermore an outermostshell color separation ink quantity set for reproducing the extendedchromatic color em lies within ink duty limits.

Extended chromatic color described above can be established in the samemanner as when increasing the number of kinds of ink. FIG. 13( b) showsextended chromatic color em in a primary color space based on inkquantities of the colors CMY. Extended chromatic color em is a colorreproducible using chromatic primary color inks C, M, Y and spot colorinks R, V.

Here, hypothetical tone values for reproducing extended chromatic colorem in the primary color space are designated CDem, MDem, and YDem, inthe color order CMY. Ink quantities of the color separation ink quantityset corresponding to the extended chromatic color em (this setcorresponds to the outermost shell color separation ink quantity set)are designated Cem, Mem, Yem, Rem, and Vem in the order CMYRV. Thus, thehypothetical CMY tone values CDem, MDem, YDem can be represented by thefollowing equations, using the substitution ink quantities indicated inFIGS. 5( b) and 5(c).

CDem=Cem+wCR×Rem+wCV×Vem

MDem=Mem+wAR×Rem+wMV×Vem

YDem=Yem+wYR×Rem+wYV×Vem  [Equation 1-1]

In this Example, the extended chromatic color em is calculated so thatthe outermost shell color separation ink quantities Cem, Mem, Yem, Rem,and Vem meet the following conditions.

(Condition 1-1) The CMYRV color separation ink quantity set meets inkduty limits.

As ink duty limits, there may be established, for example, a limit as tothe total ink quantity of all inks; limits as to ink quantities ofindividual inks; limits as to ink quantities for color mixtures of twocolors, or the like.

A limit as to the total ink quantity of all inks may be given by thefollowing equation, for example.

C+M+Y+R+V≦Duty_(—) T  [Equation 1-2]

In the Equation, C, M, Y, R and V respectively represent ink quantitiesof the colors CMYRV (this convention is also used in other equationshereinbelow). Duty_T is a limit value that has been preset withreference to type of ink and print medium.

Limits as to ink quantities of individual inks may be given by thefollowing equations, for example.

C≦Duty_C

M≦Duty_M

Y≦Duty_Y

R≦Duty_R

V≦Duty_V  [Equation 1-3]

Duty_C−Duty_V are limit values preset for each color, with reference totype of ink and print medium.

Limits as to ink quantities for color mixtures of two colors may begiven by the following equations, for example.

C+M≦Duty _(—) CM

C+Y≦Duty _(—) CY

C+R≦Duty _(—) CR

C+V≦Duty _(—) CV

M+Y≦Duty _(—) MY

M+R≦Duty _(—) MR  [Equation 1-4]

While these limits are imposed for combinations of any two inks,examples are given for six of these combinations. Duty_CM−Duty_MR arelimit values preset for combinations of inks, with reference to type ofink and print medium.

As ink duty limits, there may also be established limits regardingcombinations of any kinds of inks, such as mixtures of three colors,mixtures of four colors, etc.

Ink duty limits such as the above (Condition 1-1) can be represented byplanes (not shown) in the primary color space depicted in FIG. 13( b),using hypothetical tone values for the colors CMY derived usingsubstitution ink quantities. The area enclosed by the planes is an areain which ink duty limits are met. Thus, provided that hypothetical tonevalues for CMY for a particular color which is to be reproduced by inkquantities for the colors CMYRV lie within the area enclosed by theseplanes, each of the ink quantities meets the ink duty limits, wherebyreproduction is possible using chromatic primary color inks C, M, Y andspot color inks R, V. In this Example, a mixture of chromatic primarycolor inks C, M, Y can be substituted by spot color inks R, V inquantities smaller than total ink quantity for each ink, based on thesubstitution ink quantities indicated in FIGS. 5( b), 5(c). That is, theuse of spot color inks R, V can reproduce higher saturation withcomparable ink amount as that of a mixture of chromatic primary colorinks C, M, Y. As a result, by utilizing chromatic primary color inks CMYand spot color inks RV, it becomes possible to reproduce a wider rangeof color than that reproducible with chromatic primary color inks CMYonly.

Extended chromatic color em is depicted in FIG. 13( b). Extendedchromatic color em is located in the outer shell plane that meets inkduty limits (Condition 1-1), i.e., the reproduction color outer shellplane (not shown). Extended chromatic color em is also situated on aline passing through the origin W and the outermost shell chromaticcolor m. That is, the extended chromatic color em is situated at theintersection of the reproduction color outer shell plane with a linesegment line passing through the origin W and the outermost shellchromatic color m. In other words, the extended chromatic color em is acolor represented by the longest extended chromatic color vector havingthe same direction as the outermost shell chromatic color vectorrepresenting the outermost shell chromatic color m in the primary colorspace; furthermore an outermost shell color separation ink quantity setfor reproducing the extended chromatic color em lies within ink dutylimits. Such extended chromatic color em may be calculated by any ofvarious methods. For example, sequential approximation may be made byrepeating the processes of: selecting a color in the primary colorspace, converting chromatic primary color ink into spot color ink,calculating a color separation ink quantity set, and judging whether thecolor separation ink quantity set meets ink duty limits (Condition 1-1).Alternatively, calculation can be performed using a so-called linearprogramming method, on the basis of substitution ink quantity and inkduty limit (Condition 1-1) equations or the like. In this case theseries of steps S120-S130 (FIG. 12) would be executed one time.

In this way, by calculating extended chromatic color em whose outermostshell color separation ink quantity set meets ink duty limits (Condition1-1), extended chromatic color em situated in the same direction as theoutermost shell chromatic color and having the largest tone value can bederived within a range of good picture quality when the color isprinted.

In Step S140 in FIG. 12, calculation of a color separation ink quantityset P corresponding to input color I (FIG. 13( a)) is performed. In thisStep S140, first, calculation is performed to derive an outermost shellcolor separation ink quantity set emp for extended chromatic color em.Outermost shell color separation ink quantity set emp consists of valuespreviously calculated for the purpose of determining whether ink dutylimits (Condition 1-1) are met, when calculating extended chromaticcolor em. In the event of numerous printable inks, there is a highdegree of freedom in substitution between chromatic primary color inksand spot color inks. Thus, in some instances it may be possible toselect combinations of a plurality of types of ink in the range meetingink duty limits (Condition 1-1), for the outermost shell colorseparation ink quantity set emp corresponding to the extended chromaticcolor em. In this Example, in such instances, the combination having thesmallest total ink quantity is selected from among the plurality ofcombinations, for use as the outermost shell color separation inkquantity set emp.

Next, calculations are performed to derive a provisional colorseparation ink quantity set P on the basis of the outermost shell colorseparation ink quantity set. FIG. 13( c) is a simplified illustration ofrelationships between input color I and a provisional color separationink quantity set P. In this Example, a value obtained by multiplying theoutermost shell color separation ink quantity set emp by the ratio ofthe length LLI of the vector indicating input color I to the length LLmof the vector indicating the outermost shell chromatic color m is usedas the provisional color separation ink quantity set P. At this time,the color separation ink quantity set corresponding to the outermostshell chromatic color m is the outermost shell color separation inkquantity set emp. Any colors between the origin W and the outermostshell color separation ink quantity set emp are reproducible by thespecific combinations of print medium and ink set. Thus, the range ofcolor reproducible with a specific combination of print medium and inkset can be utilized effectively. By calculating provisional colorseparation ink quantity set P so as to be proportional to length LLI inthis way, provisional color separation ink quantity set P can becalculated readily for input color I. In addition to relationships amonginput color I and lengths LLI, LLm, provisional color separation inkquantity set P can also be calculated directly by a linear programmingmethod, on the basis of the substitution ink quantity and ink duty limit(Condition 1-1) equations, or the like. In this case, the series ofSteps S120-S140 (FIG. 12) will be executed all at once. Color to bereproduced by the provisional color separation ink quantity set Pcorresponds to a reproduction color (i.e. color to be reproduced on aprint medium) associated with the input color I.

In Step S150 in FIG. 12, calculation of a final color separation inkquantity set O is performed on the basis of the provisional colorseparation ink quantity set P. The final color separation ink quantityset O is derived by performing substitution of chromatic primary colorinks with spot color inks in accordance with substitution inkquantities. Accordingly, color reproduced by the final color separationink quantity set O will substantially match a color to be reproduced bythe provisional color separation ink quantity set P. Here, substitutionof chromatic primary color inks with spot color inks is executed inconsideration of improving graininess. Any of the processing methodsdescribed earlier (FIG. 9-FIGS. 11(a), 11(b)) or the various methodsdescribed hereinafter can be used as the process for calculating finalcolor separation ink quantity set.

Sequentially execution of the processes of Steps S100-S150 provides afinal color separation ink quantity set O expressed in a reproductioncolor system that is associated with an input color I expressed in aprimary color system. The final color separation ink quantity set Oderived in this manner can be used as the second tone value set in StepS20 of FIG. 6.

In Step S160 of FIG. 12, a decision is made as to whether final colorseparation ink quantity sets have been calculated for all input colors.In the event that calculation of all final color separation ink quantitysets has not been completed, the processes of Step S120-S150 arerepeated; if completed, the process terminates.

In order to reduce further the time required for the color separationprocess, it is preferable to limit the number of outermost shellchromatic colors for executing the series of processes. At this time, inthe event that there is no outermost shell chromatic color correspondingto an input color on which it is desired to perform a color separationprocess, a plurality of final color separation ink quantity sets thatapproximate input color can be interpolated, and a corresponding finalcolor separation ink quantity set calculated. At this time, it ispreferable to prepare in advance a number of outermost shell chromaticcolors, such that straight lines connecting outermost shell chromaticcolors and the origin W are distributed throughout the entire range ofthe color solid. By so doing, large interpolation error of a colorseparation ink quantity set in specific areas of a color solid can bereduced.

In the manner described above, in this Example, determinations ofextended chromatic color em and outermost shell color separation inkquantity set are made so as to meet the following three conditions:

(1-i) outermost shell color separation ink quantity set is within inkduty limits;(1-ii) length of the extended chromatic color vector is the greatestlength within the range reproducible by the ink set; and(1-iii) total ink quantity of the outermost shell color separation inkquantity set for reproducing the extended chromatic color em is thesmallest possible.

Even where not all of these conditions are met, the color reproductionrange can nevertheless be extended if an extended chromatic color em hashigher saturation than an outermost shell chromatic color m. Forexample, even where condition (1-ii) is not met, so that the extendedchromatic color vector is not the longest, if it is configured so as tobe longer than the outermost shell chromatic color vector, the colorreproduction range can be extended.

In order to extend color reproduction range over a wider range of hue,it is preferable to make the extended chromatic color vector longer thanthe outermost shell chromatic color vector, in a wider hue range. Here,the hue range over which an extended chromatic color vector can belengthened is a range that varies depending on hue of the useable spotcolor inks. For a spot color ink, the color reproduction range of areahaving hue close to the hue of the spot ink can be extended. Thus, byenabling use of a larger number of spot color inks of mutually differenthues, extended chromatic color vector can be made longer than theoutermost shell chromatic color vector, in a wider hue range.

As described hereinabove, in this Example, there is performed a colorseparation process that effectively utilizes a reproducible color rangeusing chromatic primary color inks and spot color inks. Thus, printingwith an extended color reproduction range can be carried out. Also,since the color separation process is carried out on the basis of anextended chromatic color situated at the intersection point of thereproduction color outer shell plane with a straight line connecting theorigin and outermost shell chromatic color, color separation processresults may be derived readily, even with an increased number of typesof useable inks.

C3. Examples 1-3 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 1 C3-1. Example 1 of Final Color Separation InkQuantity Calculation Process in Embodiment 1

In this final color separation ink quantity set calculation process, afinal color separation ink quantity set O is calculated using theprovisional color separation ink quantity set P for an input color I.The final color separation ink quantity set O is calculated such thatsubstantially identical color to provisional color separation inkquantity set P is reproduced in a range meeting ink duty limits(Condition 1-1). In order to reproduce substantially identical color toprovisional color separation ink quantity set P, a combination of inkquantities derived by substituting some ink quantities of theprovisional color separation ink quantity set P according to thesubstitution ink quantities is used as the final color separation inkquantity set O. Here, the degree of freedom of ink substitution ishigher the greater the number of types of ink that can be used. Thus,there exist a number of ink quantity combinations possible for use asthe final color separation ink quantity set O. In such instances,calculation of the final color separation ink quantity set O is carriedout with consideration to improving graininess of the image.

FIG. 15 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set in this Example. In theinitial Step S300, temporary ink quantities Rtmp, Vtmp for the spotcolor inks R, V are established. The temporary ink quantities Rtmp, Vtmpare used as indices for calculating final color separation inkquantities Ro, Vo of the spot color inks R, V in Step S310, describedlater.

FIG. 16( a) is a graph depicting a relationship between temporary inkquantity Rtmp and maximum ink quantity Rmax. FIG. 16( b) is a graphdepicting a relationship between the proportion k of temporary inkquantity Rtmp to maximum ink quantity Rmax, and maximum ink quantityRmax.

Maximum ink quantity Rmax is the maximum value that can be assumed byink quantity of red ink R when ink quantities of a provisional colorseparation ink quantity set P is substituted with red ink R on the basisof the substitution ink quantities. In this Example, maximum inkquantity Rmax represents maximum value when ink duty limits are nottaken into consideration. Accordingly, in some instances the maximumvalue of the red ink R in the final color separation ink quantities willbe smaller than maximum ink quantity Rmax. A maximum value taking inkduty limits into consideration could be used instead.

In this Example, the design is such that temporary ink quantity Rtmp issmaller, the smaller the value of maximum ink quantity Rmax, as shown inFIG. 16( a). Also, the design is such that the proportion k of temporaryink quantity Rtmp to maximum ink quantity Rmax is smaller, the smallerthe value of maximum ink quantity Rmax, as shown in FIG. 16( b). In thisexample, two specific values Rstart, Rend are set for the maximum inkquantity Rmax. Over a first range R11 in which Rmax≦Rstart, the settingis Rtmp=0 (k=0). Over a third range R13 in which Rend≦Rmax, the settingis Rtmp=Rmax (k=1). Over a second range R12 in which Rstart<Rmax<Rend,temporary ink quantity Rtmp is set so as to change in linear fashion.

These settings in relation to temporary ink quantity Rtmp may beunderstood as follows. Spot color ink is substitutable for a pluralityof chromatic primary color inks. Thus, where spot color ink is usedpreferentially, ink quantities of the plurality of chromatic primarycolor inks according to the substitution ink quantities are reduced, sothat a smaller total number of ink dots is recorded onto the printmedium. On the other hand, where chromatic primary color ink colormixtures are used preferentially, the total number of ink dots isgreater. Such difference in the number of ink dots depending on inkquantity combination is particularly marked in instances where achromatic primary color ink color mixture may be substituted by spotcolor ink in a smaller quantity than the total value of the substitutionink quantities. Also, graininess in a reproduced area (roughness of animage) tends to stand out more the smaller the number of ink dots. Thus,it is preferable in terms of improving image graininess to increase thenumber of ink dots in areas having smaller ink quantity, i.e., fewer inkdots, by means of using chromatic primary color ink mixtures in place ofspot color ink. In the example of FIGS. 16( a) and 16(b), the proportionk is designed to be smaller in association with smaller levels ofmaximum ink quantity Rmax. Thus, by making ink quantity of red ink R assmall as the temporary ink quantity Rtmp, it is possible to improvegraininess in a reproduced area.

In the example of FIG. 16( a), Rtmp is kept zero over the first rangeR11. That is, over the range of small ink quantity, temporary inkquantity Rtmp is set to a value (namely zero) that maximizes the numberof ink dots without the use of spot color ink. By setting the inkquantity of red ink R to zero in this way, standing out of ink dots ofred ink R can be avoided. Here, the first value Rstart refers to an inkquantity such that, as long as the maximum ink quantity is equal to orgreater than this value, ink dots will not readily stand out even if redink R is used. Such a first value Rstart could be established, forexample, based on a sensory test such as the following. First, agradation pattern in which red ink R ink quantity per unit of areavaries from 0 to 100% is reproduced with a mixture of chromatic primarycolor inks C, M, Y. Ink dots of red ink R are then printed atappropriate intervals in the pattern. In the small ink quantity range,red ink R dots tend to stand out, but in the large quantity range, redink R dots tend not to stand out. The ink quantity at which dots beginto not stand out can be designated as Rstart.

In the example of FIG. 16( a), the setting Rtmp=Rmax is used in thethird range R13. That is, over a range in which ink quantity isparticularly large, temporary ink quantity Rtmp is set to a value thatpreferentially uses red ink R (i.e., the maximum ink quantity Rmax). Bysetting the ink quantity for red ink R to a large value in this way,total ink quantity used can be minimized. As a result, savings inquantity of ink used are provided, and wrinkling of print media can beavoided.

The second value Rend could be established, for example, based on asensory test such as the following. First, a gradation pattern in whichred ink R ink quantity per unit of area varies from 0 to 100% isreproduced with a mixture of chromatic primary color inks C, M, Y. Inthe same manner, a gradation pattern in which red ink R quantity perunit of area varies from 0 to 100% is reproduced using red ink R. In thesmall ink quantity range, the gradation pattern produced with the colormixture has more dots and accordingly improved graininess. In the largeink quantity range, the number of ink dots is large in either of the twogradation patterns, so differences in graininess become less apparent.The two gradation patterns are compared, and the ink quantity at whichdifference in graininess between the two begins to disappear isdesignated as the second value Rend.

The maximum ink quantity Rmax can be considered as an index valueindicative of lightness of input color I, that is, a lightness parametervalue correlated to reproduction color lightness. In a bright area, theink quantity to reproduce each color is small. Thus, maximum inkquantity Rmax of red ink R is small as well. On the other hand, in adark area, the ink quantity to reproduce each color is large. Thus, inareas where red ink R is used, maximum ink quantity Rmax of red ink R islarge as well. That is, a smaller value of maximum ink quantity Rmaxindicates a lighter area, and a large value indicates a darker area.Thus, the temporary ink quantity Rtmp and proportion k given in FIGS.16( a) and 16(b) can be considered as being set so as to be smaller inassociation with higher lightness of an input color I.

Temporary ink quantity Rtmp is designed to vary continuously over theentire range for maximum ink quantity Rmax. By so doing, over gradationareas in which ink quantity varies continuously, sharp changes in inkquantity of each color component, and resulting standing out ofboundaries, can be avoided. Temporary ink quantity Rtmp is not limitedto an arrangement whereby it varies in linear fashion with respect tomaximum ink quantity Rmax; it could instead be designed to vary smoothlyusing a curve, for example.

In the example of FIG. 16( a), temporary ink quantity Rtmp of red ink Ris calculated; however, temporary ink quantity of other types of spotcolor ink can be calculated in the same manner. In any case, the designis such that the proportion of temporary ink quantity to maximum inkquantity decreases with smaller maximum ink quantity. By so doing, thenumber of ink dots can be increased by preferentially using chromaticprimary color ink in place of spot color ink so that graininess can beimproved.

In Step S310 in FIG. 15, calculation of final color separation inkquantities Ro, Vo of the spot color inks R, V is performed on the basisof the temporary ink quantities Rtmp, Vtmp of the spot color inks R, Vestablished in Step S300 described earlier. As noted, temporary inkquantities Rtmp, Vtmp are ink quantities calculated giving considerationmainly to graininess. Thus, in some instances, a combination of inkquantities using Rtmp, Vtmp may not meet ink duty limits (Condition1-1). Also, where temporary ink quantities Rtmp, Vtmp are used, in someinstances it may not be possible to reproduce the provisional colorseparation ink quantity set P derived in Step S140 of FIG. 12. In such acase, in Step S310, ink quantities close to temporary ink quantitiesRtmp, Vtmp in a range meeting these limits are used as final colorseparation ink quantities Ro, Vo.

FIGS. 17( a)-17(d) illustrate calculation of a color coordinate pointPrv(Ro, Vo) for a final color separation ink quantity pair. FIGS. 17(a)-17(d) show a two-dimensional color space representing ink quantitiesfor the colors RV as base vectors. The horizontal axis represents redink R quantity and the vertical axis represents violet ink V quantity.Ink quantity combinations of red ink R and violet ink V are show as dotsin the drawings.

FIG. 17( a) illustrates a range of values assumable by final colorseparation ink quantities Ro, Vo. Final color separation ink quantitiesRo, Vo are set within a permissible range meeting the followingconditions.

(Condition 1-1b) Final color separation ink quantities for the colorsCMYRV meet ink duty limits.(Condition 1-2b) Final color separation ink quantities for the colorsCMYRV are combinations of ink quantities that can be derived fromprovisional color separation ink quantity set P by substituting ink onthe basis of substitution ink quantities (FIGS. 5( b) and 5(c)).(Condition 1-3b) As regards color components with temporary ink quantitysmaller than provisional color separation ink quantity, final colorseparation ink quantity is equal to or less than provisional colorseparation ink quantity.

“Condition 1-1b” is the same as “Condition 1-1” described earlier. Thiscondition can be represented by straight lines in FIG. 17( a).Accordingly, the permissible range can be represented by an areaenclosed by lines corresponding to this condition. In the example ofFIG. 17( a), to simplify the description, an area enclosed by fivestraight lines LR-LRVM2 described hereinbelow will be designated aspermissible area RA.

Straight line LR is a line corresponding to the upper limit of red inkR. The ink quantity of red ink R can be increased by substituting eachink according to substitution ink quantities (FIG. 5). However, theupper limit value therefor is subject to the restriction of the ink dutylimit for red ink R, and to the restriction of the maximum ink quantityfor red ink R which is determined by provisional color separation inkquantity set P and the substitution ink quantities. Straight line LRcorresponds to the upper limit meeting these restrictions.

Straight line LV is a line corresponding to the upper limit for violetink V. The meaning of this line is the same as for line LR describedabove.

Straight line LVC is a line corresponding to the lower limit for violetink V. It is possible to decrease the ink quantity of violet ink V bysubstituting the same with the two chromatic primary color inks C, M;this, however, increases the ink quantities of the chromatic primarycolor inks. Accordingly, the lower limit for violet ink V is subject tothe restriction of ink duty limits for the chromatic primary color inks.Straight line LVC corresponds to the lower limit meeting theserestrictions.

Straight line LVRM1 is a line corresponding to a common lower limit forthe two inks R, V. Ink quantities of red ink R and violet ink V can bothbe decreased by substitution with two chromatic primary color inks.Instead of this, the ink quantity of the principal component primarycolor ink common to both inks R, V (FIGS. 5( b), 5(c); in this example,magenta M) increases. The ink quantity of magenta ink M is subject tothe restrictions of the ink duty limit for magenta ink M, and themaximum ink quantity for magenta ink M determined by provisional colorseparation ink quantity set P and substitution ink quantities.Accordingly, the lower limit for ink quantity of the two inks R, V isrestricted to a range in which it is possible to share the upper limitsfor magenta ink M quantity with one another. Under such a restriction,lower limits for the two inks R, V are inversely proportional. Straightline LRVM1 corresponds to the lower limit of the two inks R, Vdetermined in this manner.

Straight line LVRM2 is a line corresponding to a common upper limit forthe two inks R, V. Ink quantities of red ink R and violet ink V can bothbe increased by substitution them for two chromatic primary color inks.Instead of this, the ink quantity of the principal component primarycolor ink common to both inks R, V (magenta M) decreases. The upperlimit for ink quantity of the two inks R, V is restricted to a range inwhich it is possible to share the maximum ink quantity of magenta ink Mwith one another. Under such a restriction, upper limits for the twoinks R, V are inversely proportional. Straight line LRVM2 corresponds tothe upper limit of the two inks R, V determined in this manner.

In this Example, among the spot color inks, the one whose temporary inkquantity is less than its provisional color separation ink quantity aresubject to the restriction that its final color separation ink quantityis equal to or less than its provisional color separation ink quantity(Condition 1-3b). For example, in the event that the temporary inkquantity Rtmp of red ink R is smaller than the provisional colorseparation ink quantity Rp, final color separation ink quantity Ro islimited to a value equal to or less than the provisional colorseparation ink quantity Rp. A small temporary ink quantity means thatadjusting the ink quantity of that color to a low level is desirable interms of improving graininess. Of a plurality of color separation inkquantity sets derived by substituting some ink quantities according tothe substitution ink quantities on the basis of provisional colorseparation ink quantity sets, those color separation ink quantity setshaving small ink quantities of spot color inks will have less readilyapparent graininess, due to the greater total number of ink dots.Accordingly, for inks whose temporary ink quantity is smaller than theprovisional color separation ink quantity, final color separation inkquantity is prevented from becoming larger than provisional colorseparation ink quantity.

In Step S310 in FIG. 15, in permissible range RA derived in the abovemanner, a combination of ink quantities close to the color coordinatepoint Prv(Rtmp, Vtmp) of the temporary ink quantity pair are used as thefinal color separation ink quantities Ro, Vo. The following descriptionof calculation of final color separation ink quantities Ro, Vo assumesthe relationship between permissible range RA and temporary ink quantitypair color coordinate point Prv(Rtmp, Vtmp) to fall under three separatecases.

(Case 1-1) The temporary ink quantity pair color coordinate point Prv islocated inside permissible range RA.(Case 1-2) The temporary ink quantity pair color coordinate point Prv islocated outside permissible range RA, and a line connecting the origin Wand the temporary ink quantity pair color coordinate point Prv passesthrough permissible range RA.(Case 1-3) The temporary ink quantity pair color coordinate point Prv islocated outside permissible range RA, and a line connecting the origin Wand the temporary ink quantity pair color coordinate point Prv does notpass through permissible range RA.

(Case 1-1):

FIG. 17( b) depicts a case in which temporary ink quantity pair colorcoordinate point Prv is located inside permissible range RA. In thiscase, temporary ink quantities Rtmp, Vtmp are used as-is as final colorseparation ink quantities Ro, Vo. By so doing, final color separationink quantities Ro, Vo that are favorable in terms of improvinggraininess can be obtained. In the example of FIG. 17( b), since theviolet ink V temporary ink quantity Vtmp is smaller than the provisionalcolor separation ink quantity Vp, the permissible range RA is restrictedto the range Vo≦Vp.

(Case 1-2):

FIG. 17( c) depicts a case in which temporary ink quantity pair colorcoordinate point Prv is located outside permissible range RA, and astraight line LP connecting the origin W and the temporary ink quantitypair color coordinate point Prv passes through permissible range RA. Inthis case, an ink quantity combination represented by, of theintersection points of line LP with the boundaries of permissible rangeRA, the point Oa closest to the temporary ink quantity pair colorcoordinate point Prv is used for the final color separation inkquantities Ro, Vo. Line LP is a straight line maintaining a constantratio of temporary ink quantities Rtmp, Vtmp. By using a point on such aline LP, ink quantity of one of the spot color inks R, V can be reducedappropriately, while preventing the other ink quantity from not becomingsufficiently small. In other words, by using a point on line LP, finalcolor separation ink quantities Ro, Vo can be calculated while takinginto consideration adverse effects of each spot color ink on graininess.

The ink combination used as the final color separation ink quantity setis not limited to the combination represented by point Oa describedabove. Any combination represented by a point situated at a locationinside permissible range RA and close to temporary ink quantity paircolor coordinate point Prv can improve graininess. For example, an inkquantity combination represented by the intersection point Ob of aboundary of permissible range RA with a straight line connectingprovisional color separation ink quantity set P and temporary inkquantity pair color coordinate point Prv could be used. It is alsopossible to use a combination represented by a point Oc situated at alocation inside permissible range RA, and closest to temporary inkquantity pair color coordinate point Prv. In any event, by using acombination of ink quantities represented by a point situated at alocation inside permissible range RA and close to temporary ink quantitypair color coordinate point Prv, calculation of final color separationink quantities Ro, Vo can be carried out in consideration of improvinggraininess.

(Case 1-3):

FIG. 17( d) depicts a case in which temporary ink quantity pair colorcoordinate point Prv is located outside permissible range RA, and astraight line LP connecting the origin W and the temporary ink quantitypair color coordinate point Prv does not pass through permissible rangeRA. In this case, an ink quantity combination represented by a pointOd—which is a point inside the permissible range RA that minimizes thefinal color separation ink quantity Ro relating to the red ink R which,of the temporary ink quantities Rtmp, Vtmp, has the smaller valueRtmp—is used as the final color separation ink quantities Ro, Vo. Inkhaving the smallest temporary ink quantity has the greatest effect ongraininess. Accordingly, by using the point Od at which the ink quantityof this ink is minimized, it is possible to improve graininess in thereproduced image area.

The combination of inks used as the final color separation ink quantityset is not limited to the combination represented by the aforementionedpoint Od. Any combination represented by a point situated inside thepermissible range RA and close to the temporary ink quantity pair colorcoordinate point Prv can improve graininess.

In the event that all temporary ink quantities Rtmp, Vtmp of the spotcolor inks are zero, graininess can be improved with any combinationrepresented by a point situated inside the permissible range RA andclose to the origin W. As such a point, it would be possible, forexample, to use the intersection point (not shown) of a permissiblerange RA boundary with a straight line connecting the color coordinatepoint P of the provisional color separation ink quantity set and theorigin W. A point that minimizes distance from the origin W may also beused.

In Step S320 in FIG. 15, calculation of a final color separation inkquantity set for all inks making the ink set is performed. The inkquantities calculated in Step S310 are used as the final colorseparation ink quantities Ro, Vo for the spot color inks. Final colorseparation ink quantities Co, Mo, Yo for the chromatic primary colorinks are calculated on the basis of the provisional color separation inkquantity set P and substitution ink quantities.

As described hereinabove, the final color separation ink quantity setcalculation process of this Example is carried out in consideration ofimproving graininess. Where an image area it is desired to reproduce isbright, ink quantity of each color of ink is smaller. At this time,since less spot color ink is used and more chromatic primary color inkis used, graininess can be improved.

In an area of particularly high lightness, the magnitude of each inkquantity of an ink set is smaller relative to ink duty limits.Accordingly, ink duty limits are met by a combination of greater inkquantities. As a result, where temporary ink quantities Rtmp, Vtmp ofthe spot color ink are zero, final color separation ink quantities Ro,Vo can be brought to zero. That is, in an area of particularly highlightness, color is reproduced without the use of spot color ink, andthus in such particularly bright areas, standing out of ink dots of spotcolor ink can be avoided.

In this Example, the final color separation ink quantity set calculationprocess is performed according to temporary ink quantities Rtmp, Vtmpestablished independently for each of the spot color inks, i.e.depending on the effect of each ink on graininess. As a result,graininess can be improved appropriately in consideration of effect ofeach spot color ink on graininess.

This sort of final color separation ink quantity set calculation processmay be carried out in similar fashion even where more types of spotcolor ink are used. For example, where three spot color inks are used,first, a temporary ink quantity is calculated for each ink. Next, acombination of ink quantities represented by a point situated within apermissible range (represented by a solid in a color space in which thethree ink quantities are represented by base vectors) and close to apoint represented by a temporary ink quantity set is used as a finalcolor separation ink quantity set of spot color ink. In this case aswell, where a final color separation ink quantity set is calculatedaccording to the positional relationship of the permissible range and astraight line passing through a point represented by a temporary inkquantity set, the final color separation ink quantity set can becalculated in consideration of the balance of effect of each spot colorink on graininess.

As described hereinabove, the final color separation ink quantity setcalculation process of this Example is carried out in consideration ofimproving graininess, whereby roughness of images in bright areasthereof can be reduced.

C3-2. Example 2 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 1

A point of difference from Example 1 described hereinabove is thattemporary ink quantity Rtmp is set depending on lightness L of a colorto be reproduced. FIG. 18 is a graph showing a relationship of lightnessL to the proportion k of temporary ink quantity Rtmp to maximum inkquantity Rmax.

Where provisional color separation ink quantity set P is expressed in aprimary color space (FIG. 13( a)), for example, the distance between apoint produced by projecting the point P onto the achromatic color axis(the straight line connecting the origin W and a point K) and the originW may be used as lightness L of the color to be reproduced. Here,greater distance from the origin W indicates lower lightness. As themaximum ink quantity Rmax, there may be used the maximum value assumableby the value of ink quantity of red ink R, among combinations of inkquantities able to reproduce color substantially identical to the colorto be reproduced by provisional color separation ink quantity set P.

In this Example, as shown in FIG. 18, the proportion k of temporary inkquantity Rtmp to maximum ink quantity Rmax is set so as to be smallerthe higher the lightness L of the color to be reproduced, i.e. thebrighter it is. Temporary ink quantity for the other spot color ink isset similarly. Accordingly, in an image area of high lightness L andsmall ink quantities of each ink, less spot color ink is used, andchromatic primary color ink is used preferentially to increase thenumber of ink dots, so that graininess can be improved.

In the brightest range R21 wherein Lstart≦L, the setting Rtmp=0 is used.As a result, in areas of particularly high lightness, colors arereproduced without the use of spot color ink, so that standing out ofink dots of spot color ink can be prevented. In the darkest range R23wherein L≦Lend, the setting Rtmp=Rmax is used. As a result, in areas ofparticularly low lightness, colors are reproduced using spot color inkpreferentially, thereby providing savings in quantity of ink used, andalso preventing wrinkling of the print medium.

The values Lstart, Lend used to indicate specific magnitudes oflightness L can be set in the same manner as Rstart, Rend in FIG. 16(a). For example, they can be set on the basis of a sensory test bycomparing a spot color ink gradation pattern reproduced while varyinglightness L from minimum to maximum value, with a gradation patternproduced by mixture of chromatic primary color inks.

In this Example, as regards violet ink V, temporary ink quantity Vtmpdepending on lightness L is established using the same method as for redink R. Where even more spot color inks are used, temporary inkquantities can be established in similar fashion.

C3-3. Example 3 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 1

FIG. 19 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set in this Example. A point ofdifference from the preceding Examples is that the range of possiblevalues of final color separation ink quantities of spot color inks islimited depending on lightness of input color (or reproduction color).In this Example, final color separation ink quantities Ro, Vo areestablished within a permissible range meeting the following conditions.

(Condition 1-1c) Final color separation ink quantities for the colorsCMYRV meet ink duty limits.(Condition 1-2c) Final color separation ink quantities for the colorsCMYRV are combinations of ink quantities that can be derived fromprovisional color separation ink quantity set P by substituting inks onthe basis of substitution ink quantities.(Condition 1-4c) The sum of final color separation ink quantities ofspot color inks is equal to or less than a spot color ink quantity limitvalue T2.

Conditions 1-1c, 1-2c are the same as conditions 1-1b, 1-2b describedpreviously. The addition of Condition 1-4c is a point of difference fromthe earlier Examples.

In Step S400 in FIG. 19, a spot color ink quantity limit value T2 isestablished. FIG. 20 is a graph of the relationship between reproductioncolor lightness L and the proportion k2 of limit value T2 to the maximumvalue T2max assumable by the sum of final color separation inkquantities of spot color inks; the sum corresponds to a spot color inkquantity parameter in the invention.

As the maximum value T2max there may be used the maximum value possiblefor the sum of ink quantities of spot color inks R, V, amongcombinations of ink quantities able to reproduce color substantiallyidentical to the color to be reproduced by provisional color separationink quantity set P (for example S510 in FIG. 8 or S140 in FIG. 12).Alternatively, the maximum value possible for the sum of ink quantitiesof spot color inks R, V, among combinations of ink quantities able toreproduce lightness substantially identical to the lightness L ofreproduction color to be reproduced by provisional color separation inkquantity set P.

A point of difference from the example of FIG. 18 is that instead ofestablishing a temporary ink quantity for each of the spot color inks R,V, an upper limit (limit value) T2 is established for the sum of inkquantities of spot color inks R, V (spot color ink quantity parameter).In the example of FIG. 20, proportion k2 is set so as to be smaller thehigher the lightness L of the reproduction color, i.e. the brighter itis. Accordingly, in an image area of high lightness L and small inkquantities of each ink, the sum of ink quantities of spot color ink issmaller, and chromatic primary color ink is used preferentially toincrease the number of ink dots, so that graininess can be improved.

In the brightest range R31 wherein Lstart≦L, the setting k2=0, i.e.T2=0, is used. As a result, in areas of particularly high lightness,colors are reproduced without the use of spot color inks, so thatstanding out of ink dots of spot color inks can be prevented. In thedarkest range R33 wherein L≦Lend, the setting k2=1, i.e., T2=T2Max, isused. As a result, in areas of particularly low lightness, colors arereproduced using spot color inks preferentially, thereby providingsavings in quantity of ink used, and also preventing wrinkling of theprint medium.

In Step S410 of FIG. 19, calculation of final color separation inkquantities Ro, Vo of spot color inks R, V is performed on the basis ofthe spot color ink limit value T2 established in the aforementioned StepS400 of FIG. 19. In this Example, final color separation ink quantitiesRo, Vo are determined on the basis of provisional color separation inkquantity P and spot color ink limit value T2, without calculating atemporary ink quantity pair Rtmp, Vtmp (FIG. 15, S300).

FIGS. 21( a)-12(c) illustrate calculation of final color separation inkquantities Ro, Vo of spot color inks. In the drawing, area RA is apermissible area meeting the aforementioned Conditions 1-1c and 1-2c.Also shown in the drawing is a straight line LT2 representing theaforementioned Condition 1-4c. A permissible area RB meeting the threeconditions 1-1c, 1-2c, and 1-4c is indicated by hatching.

FIG. 21( a) depicts an instance in which a provisional color separationink quantity set coordinate point P lies within permissible area RB. Inthis case, provisional color separation ink quantities Rp, Vp are usedas final color separation ink quantities Ro, Vo.

FIG. 21( b) depicts an instance in which a provisional color separationink quantity set coordinate point P lies outside permissible area RB. Inthis case, an ink quantity combination represented by, of theintersection points of a straight line connecting provisional colorseparation ink quantity set coordinate point P to origin W, with theboundaries of permissible range RB, the point Oe closest to theprovisional color separation ink quantity set coordinate point P is usedfor the final color separation ink quantities Ro, Vo. In this way, usinga combination of ink quantities inside a permissible area RB that meetscondition 1-4c, graininess can be improved.

The ink combination used as the final color separation ink quantity setis not limited to the combination represented by point Oe describedabove; any combination represented by a point situated insidepermissible range RB can improve graininess.

FIG. 21( c) depicts an instance in which there is no permissible area RBthat meets all of the conditions 1-1c, 1-2c, and 1-4c. In this case, anink quantity combination represented by, of the intersection points of astraight line connecting provisional color separation ink quantity setcoordinate point P to origin W, with the boundaries of permissible rangeRA, the point Of closest to the origin W is used for the final colorseparation ink quantities Ro, Vo. By using a point close to the originin this way, graininess can be improved.

The ink combination used as the final color separation ink quantity setis not limited to the combination represented by point Of describedabove; any combination represented by a point situated insidepermissible range RA and close to the origin W can improve graininess.

In Step S420 of FIG. 19, calculation of a final color separation inkquantity set of all inks making up the ink set is performed. The inkquantities calculated in Step S410 are used as final color separationink quantities Ro, Vo for the spot color inks. Final color separationink quantities Co, Mo, Yo for the chromatic primary color inks arecalculated on the basis of the provisional color separation ink quantityset P and substitution ink quantities.

In the manner described above, in the final color separation inkquantity set calculation process in this Example, a restriction on spotcolor ink quantity parameter is adjusted depending on input color (orreproduction color) lightness. Accordingly, since the possible range ofvalues assumed by final color separation ink quantities of spot colorinks is restricted depending on lightness of the input color (orreproduction color), final color separation ink quantities can becalculated with consideration to improving graininess. Thelightness-dependent permissible range restriction is not limited toCondition 1-4c; any restriction of permissible range to a smaller rangeof ink quantity with lighter input color (or reproduction color) wouldbe acceptable. For example, in a color space in which spot color inkquantities are represented as base vectors, distance from the origin toa point represented by the final color separation ink quantities Ro, Vocould be used as a spot color ink quantity parameter. By imposing onthis parameter the restriction of being smaller in association withgreater lightness of input color (or reproduction color), graininess canbe improved. Additionally, a weighted average value or weighted averagedistance, weighted according to the tendency of ink dots of ink of eachcolor to stand out, could be used as a spot color ink quantityparameter, restricting the magnitude thereof depending on lightness. Thespot color ink quantity parameter is not limited to sum of inkquantities, distance, weighted average value, or weighted averagedistance as mentioned above; any characteristic that increases inmagnitude with greater ink quantities of spot color inks would beacceptable. In this way, by imposing a condition that limits a spotcolor ink quantity parameter to a smaller range in association withgreater lightness as represented by a lightness parameter value, it ispossible to improve graininess more appropriately. This final colorseparation ink quantity set calculation process can be carried out insimilar fashion even when a greater number of spot color inks are used.

As the final color separation ink quantity set calculation process S150of the color separation process of this Example (FIG. 12), the method(FIG. 9-FIG. 11) employed in the final color separation ink quantity setcalculation process S520 of the color separation process shown in FIG. 8may be used instead of the various methods depicted in FIGS. 15-17, FIG.18, and FIGS. 19-21. Similarly, as the color separation ink quantity setcalculation process S520 of the color separation process shown in FIG.8, any of the various methods (FIGS. 15-17, FIG. 18, and FIGS. 19-21)employed in the final color separation ink quantity set calculationprocess S150 of the color separation process of this Example (FIG. 12)may be used instead of the method depicted in FIG. 9-FIG. 11.

C4. Example 3 of Color Separation Process in Embodiment 1

FIG. 22 is a flowchart depicting a processing routine of a third Exampleof the color separation process in Embodiment 1. A point of differencewith the color separation process example of FIG. 12 earlier is that anunder color removal (UCR) process S220 using black ink K is executed.The UCR process of this Example is a process wherein some chromaticprimary color ink C, M, Y tone values are substituted with black ink Ktone values. As the UCR process may be implemented by various widelyknown methods, a detailed description will not be provided here.

In Step S200, an ink set composed of the chromatic primary color inks C,M, Y, the spot color inks R, V, and black ink K is established as theuseable ink set.

Next, in Step S210, ink duty limits, which are limits as to ink quantityfor each color of the ink set, are established. A difference with theink duty limits in the Example shown in FIG. 12 is that they areestablished with consideration to the ink quantity of black ink K(described in detail later). Next, in Step S220, the UCR process isexecuted on an input color targeted for color separation processing (forexample, in Step S20 in FIG. 6, one represented by a primary color tonevalue set). As a result, an input color I represented by tone values Ci,Mi, Yi, Ki of the colors CMYK is obtained. Of these tone values, tonevalues Ci, Mi, Yi of the colors CMY are subjected to a color separationprocess using extended chromatic color em. The series of processesS230-S270 is the same as processes S120-S160 of the Example shown inFIG. 12. As a result, there are obtained color separation ink quantitiesCo, Mo, Yo, Ro, Vo for the tone values Ci, Mi, Yi of the colors CMY. Forthe black ink K, the tone value Ki obtained as the result in UCR process220 is used as the color separation ink quantity Ko.

In this way, in the color separation process of Example 3, there isperformed a color separation process that effectively utilizes the rangeof color possible to reproduce using black ink K in addition tochromatic primary color inks CMY and spot color inks RV. This makes itpossible to perform printing with an additionally extended colorreproduction range.

In this Example, it is preferable for the ink duty limit (Condition 1-1)described earlier to also include a limit that takes into considerationink quantity of black ink K. For example, as regards limits on quantityof all inks, given in Equation 1-2, the sum of ink quantities of thecolors CMYRV in conjunction with the ink quantity Ki of black ink Kderived in Step S220 can be set so as to be equal to or less thanDuty_T. By so doing, ejection of ink in quantities exceeding the limitof ink absorbency of the print medium can be avoided. As regards limitson ink quantity in the case of mixtures of several colors as well,limits can be established using ink quantity Ki of black ink K. Asregards limits on ink quantity of black in K alone, these are preferablytaken into consideration when calculating ink quantity Ki during the UCRprocess 220.

When the color separation process of this Example is implemented in StepS20 of the color conversion lookup table creation process illustrated inFIG. 6, the second tone value set consists of tone values represented byink quantities of the chromatic primary color inks CMY, spot color inksRV, and black ink K. Thus, in Step S30, color patches reproduced usingthe colors CMYRVK are created.

D. Embodiment 2

The device arrangement described in FIG. 1-FIG. 5 and the colorconversion lookup table creation process described in FIGS. 6 and 7 mayalso be implemented in Embodiment 2. Embodiment 2 differs fromEmbodiment 1 in terms of the color separation process method, describedhereinbelow.

D1. Details of Example 1 of Color Separation Process in Embodiment 2

FIG. 23 is a flowchart depicting the processing routine of Example 1 ofthe color separation process in Embodiment 2. Steps S1500-S1530 in FIG.23 are similar to Steps S500-S530 in FIG. 8, with the exception that thedetails of the routine of Step S1520, described hereinbelow, differ fromthe details of the routine of Step S520 (FIG. 9).

FIG. 24 is a flowchart depicting the details of the routine of StepS1520. In Step S1600, residual ink quantities RES_R, RES_V correspondingrespectively to the two spot color inks R, V are determined from theprovisional color separation ink quantity set P. Residual ink quantityis the sum of ink quantities of inks other than the spot color inks. Inthe next Step S1610, temporary ink quantities Rtmp, Vtmp of the spotcolor inks R, V are determined using residual ink quantities RES_R,RES_V.

FIGS. 25( a)-25(e) illustrate relationships between spot color ink usagerate and ink quantities of other inks. In the example of FIGS. 25(a)-25(e), in order to simplify the description, it is assumed that twotypes of ink, namely cyan ink C and magenta ink M, are utilizable aschromatic primary color inks, and one type of ink, namely violet ink V,is utilizable as spot color ink.

FIG. 25( a) illustrates the range of hue reproducible using cyan ink Cand magenta ink M. Violet ink V has a hue between cyan ink C and magentaink M. In FIG. 25( a), two colors ColA, ColB are shown. The two colorsColA, ColB have hues between violet ink V and cyan ink C. Color ColA hasa hue close to that of violet ink V. Color ColB is a color derived byadjusting the hue of color ColA closer to cyan ink C.

FIG. 25( b) illustrates one example of ink quantities of the three inksC, M, V for reproducing color ColA. Since color ColA has a hue close toviolet ink V, it is possible to reproduce using a large amount of violetink V. Also, since color ColA has a hue between violet ink V and cyanink C, it is reproduced with some cyan ink C. In the example of FIG. 25(b), cyan ink C quantity is 2, magenta ink M quantity is 0, and violetink V quantity is 2. Here, ink quantity of each of the colors CMV canassume a value in the range 0-100. Ink quantity=0 means that an inkquantity is zero; ink quantity=100 means the ink quantity used toreproduce a solid area.

Violet ink V is characterized by higher saturation and density than cyanink C or magenta ink M. Accordingly, in bright areas, if violet ink Vare used preferentially as in FIG. 25( b), the ink dots would tend tostand out, with the possibility that graininess may not be adequatelyimproved.

FIG. 25( c) illustrates another example of ink quantities of the threeinks C, M, V for reproducing color ColA. The difference from FIG. 25( b)is that the violet ink V quantity is smaller, and the cyan ink C andmagenta ink M quantities are larger. Specifically, cyan ink C is 11,magenta ink M is 9, and violet ink V is 11.

Here it is assumed that a mixture of equal quantities of cyan ink C andmagenta ink M can reproduce hue and saturation substantially identicalto those of an equivalent quantity of violet ink V. That is,substitution quantities for violet ink V are cyan ink C=1 and magentaink M=1. The example of FIG. 25( c) is a combination of ink quantitiesderived on the basis of ink quantities in FIG. 25( b), by substitutingan violet ink V in ink quantity=9 with cyan ink C in ink quantity=9 andmagenta ink M in ink quantity=9. Accordingly, the example of FIG. 25( c)can reproduce a color ColA substantially identical to that of theexample of FIG. 25( b). There are innumerable combinations of inkquantities that can be derived by substituting ink quantities accordingto substitution ink quantities. Of this plurality of combinations, theexample of FIG. 25( b) depicts the combination having the largest inkquantity of violet ink V.

Where the ink quantity of violet ink V is reduced as in FIG. 25( c), inkquantities of the plurality of other inks C, M increase, making itunlikely that violet ink V dots will stand out. Here, it is assumed thatwhere the total ink quantity of the other inks C, M is double or morethe ink quantity of violet ink V, it is unlikely that violet ink V dotswill stand out. Accordingly, by setting the violet ink V quantity usagerate (in this example, the proportion of ink quantity to maximum value)to 55% (=11/20) as in FIG. 25( c), it is possible to improve graininess.

FIG. 25( d) illustrates an example of ink quantities of the three inksC, M, V for reproducing color ColB. Since color ColB is closer in hue tocyan ink C than is color ColA, cyan ink C is used in relatively largeproportion even in cases where violet ink V is used preferentially. Inthe example of FIG. 25( d), cyan ink C quantity is 10, magenta ink Mquantity is 0, and violet ink V quantity is 15. In this example, as inFIG. 25( b), the combination having the largest violet ink V quantity isshown.

In the example of FIG. 25( d), the sum of quantities of inks other thanviolet ink V, which makes ten, is smaller than a value that is doublethe ink quantity of violet ink V, which makes 30. Accordingly, in brightareas, violet ink V ink dots will tend to stand out, possibly precludingadequate improvement of graininess.

FIG. 25( e) illustrates another example of ink quantities of the threeinks C, M, V for reproducing color ColB. The difference from FIG. 25( d)is that the violet ink V quantity is smaller, and the cyan ink C andmagenta ink M quantities are larger. Specifically, cyan ink C quantityis 15, magenta ink M quantity is 5, and violet ink V quantity is 10.These ink quantities are a combination of ink quantities derived on thebasis of ink quantities in FIG. 25( d), by substituting an violet ink Vin quantity=5 with cyan ink C in quantity=5 and magenta ink M inquantity=5.

In the example of FIG. 25( e), the sum of quantities of inks other thanviolet ink V, which makes 20, is about double the ink quantity of violetink V, which makes 10. Accordingly, as relates to color ColB, graininesscan be improved where the violet ink V ink quantity usage rate is 67%(=10/15).

In this way, as relates to a color ColA of hue relatively close to thatof a spot color ink, reproduction using the spot color inkpreferentially results in smaller quantities of other inks. Accordingly,to improve graininess, it is desirable to have a small usage rate ofspot color ink. As relates to a color ColB of hue relatively close tothat of a chromatic primary color ink, on the other hand, even wherereproduction uses the spot color ink preferentially, ink quantities ofother inks maintain relatively large values. Accordingly, graininess canbe improved even with the spot color ink usage rate set to a relativelyhigh value. Also, where spot color ink usage rate is set to a relativelyhigh value, total ink quantity of all inks is smaller than when set to alow usage rate, thereby providing savings in ink usage.

Incidentally, this can be considered in terms of the relationshipbetween spot color ink usage rate and quantities of inks other than spotcolor ink, instead of the relationship between spot color ink usage rateand hue. As shown in FIGS. 25( a)-25(e), when hue is relatively close toa chromatic primary color ink, quantities of other inks maintainrelatively large values even with a large ink quantity of spot colorink. On the other hand, when hue is relatively close to a spot colorink, a larger quantity of spot color ink results in smaller quantitiesof other inks. Accordingly, by increasing the spot color ink usage ratewhen quantities of other inks are large and reducing the spot color inkusage rate when quantities of other inks are small, graininess can beimproved appropriately depending on hue.

The relationship between spot color ink usage rate and quantities ofinks other than spot color ink discussed above can be consideredanalogously even when the number of inks is increased. In such instancesas well, in preferred practice the spot color ink usage rate whenquantities of other inks are small will preferably be set smaller thanthe usage rate when quantities of other inks are large. By so doing,graininess in bright areas can be improved, and savings in ink usageobtained.

In Step S1600 in FIG. 24, residual ink quantities RES_R, RES_Vcorresponding to the spot color inks R, V are calculated. In thisExample, residual ink quantities RES_R, RES_V are represented by thefollowing equations, using ink quantities included in the provisionalcolor separation ink quantity set P calculated in Step S1510 of FIG. 23.

RES _(—) R=Cp+Mp+Yp+Vp

RES _(—) V=Cp+Mp+Yp+Rp  [Eq. 2-1]

In the equations, Cp, Mp, Yp, Rp and Vp are ink quantities included inthe provisional color separation ink quantity set P. The residual inkquantity RES_R corresponding to red ink R is the sum of quantities ofinks other than red ink R. The residual ink quantity RES_V correspondingto violet ink V is the sum of quantities of inks other than violet inkV.

Next, in Step S1610, temporary ink quantities Rtmp, Vtmp for the spotcolor inks R, V are determined from the residual ink quantities RES_R,RES_V and the provisional color separation ink quantity set P. FIG. 26(a) shows a graph for calculating temporary ink quantity Rtmp from theprovisional color separation ink quantity Rp of red ink R. Graph L10 isa graph corresponding to a relatively large residual ink quantityRES_R10, while graph L20 is a graph corresponding to a relatively smallresidual ink quantity RES_R20. For example, graph L10 corresponds to thecase of FIGS. 25( d) and 25(e), and graph L20 corresponds to the case ofFIGS. 25( b) and 25(c).

Following is a description of the relationship between temporary inkquantity Rtmp and provisional color separation ink quantity Rp, takingthe example of graph L10. Over a first range R1 in which provisionalcolor separation ink quantity Rp goes from zero to a first value Rs10,temporary ink quantity Rtmp is set to zero. Over a second range R2 inwhich provisional color separation ink quantity Rp goes from first valueRs10 to a second value Re10, temporary ink quantity Rtmp rises in linearfashion from zero. However, in second range R2 the temporary inkquantity Rtmp is maintained at a value smaller than the provisionalcolor separation ink quantity Rp. Over a third range R3 in whichprovisional color separation ink quantity Rp is equal to or greater thansecond value Re10, temporary ink quantity Rtmp is set a value equal tothe provisional color separation ink quantity Rp.

As in graph L10, in graph L20 corresponding to a relatively smallresidual ink quantity RES_R20, temporary ink quantity Rtmp is setaccording to a first value Rs20 and a second value Re20. However, thefirst value Rs20 is set to a value greater than the first value Rs10 ofgraph L10, and the second value Re20 is set to a value greater than thesecond value Re10 of graph L10. Accordingly, for a given value ofprovisional color separation ink quantity Rp, a relatively largetemporary ink quantity Rtmp can be derived when residual ink quantityRES_R is relatively large.

FIG. 26( b) is a graph showing the proportion k of temporary inkquantity Rtmp with respect of provisional color separation ink quantityRp. Taking the example of the proportion k10 corresponding to residualink quantity RES_R10, as will be understood from the graph, proportionk10 is zero in first range R1, and in second range R2 increases at aconstant rate from zero to 1. In third range R3, proportion k10 isconstant at 1.

As regards the proportion k20 corresponding to residual ink quantityRES_R20 as well, the manner of change of proportion k20 in each of thefirst to third ranges R201-R203 is similar to the manner of change ofproportion k10. For a given value of provisional color separation inkquantity Rp, a relatively large proportion k can be derived whenresidual ink quantity RES_R is relatively large.

The reason that the manner of change of temporary ink quantity Rtmprelative to change of provisional color separation ink quantity Rp isset as shown in FIGS. 26( a) and 26(b) is described hereinbelow, takingthe example of graph L10. When provisional color separation ink quantityRp is small, an image reproduced with reproduction color thereof willfrequently be a so-called highlight area (high lightness area). The spotcolor inks R, V are characterized by higher saturation and density thanthe chromatic primary color inks. Since dots of spot color inks havehigh saturation and density, they tend to stand out in highlight areas,with the possibility of exacerbating graininess of the image.Accordingly, in highlight areas, it is preferable for quantities of spotcolor inks to be small. Since decreasing quantities of spot color inksalso increases quantities of chromatic primary color inks, total inkquantity increases as well. In this aspect as well, graininess of animage can be improved by decreasing quantities of spot color inks inhighlight areas. Accordingly, in first range R1 in FIG. 26( a),formation of dots of spot color ink in highlight areas is avoided bysetting the spot color ink quantity Rtmp to zero. By so doing,graininess of an image in highlight areas can be markedly improved.

On the other hand, in third range R3 wherein provisional colorseparation ink quantity Rp is a large value, it is preferable toincrease the quantity of spot color ink, so as to increase thesaturation of the image. Accordingly, in the range R3, the provisionalcolor separation ink quantity Rp is used as-is as the temporary inkquantity Rtmp. In the second range R2, temporary ink quantity Rtmpincreases in linear fashion so that temporary ink quantity Rtmp does notchange in stepwise fashion over the three ranges R1-R3, but ratherchanges smoothly. Alternatively, temporary ink quantity Rtmp may changealong a curve, instead of changing in linear fashion. Typically, it ispreferable for temporary ink quantity Rtmp to change continuously at aconstant rate of increase.

Since a red component of provisional color separation ink quantity Rprelating to a given reproduction color represents the density of the redink component of that reproduction color, this ink quantity Ro can bethought of as an index indicating lightness for the red ink component.When the red component of provisional color separation ink quantity Rprelating to a given reproduction color is large, lightness of thereproduction color tends to be low. Accordingly, the provisional colorseparation ink quantity Rp can be thought of as a lightness parametervalue correlated to lightness of the reproduction color.

The reason that the manner of change of temporary ink quantity Rtmprelative to change of residual ink quantity RES_R is set as shown inFIGS. 26 (a) and 26(b) is as follows. In many instances, when residualink quantity RES_R is small, the hue of the reproduction color thereofis often close to that of a spot color ink. Where such reproductioncolor is reproduced using spot color ink preferentially, quantities ofthe plurality of chromatic primary color inks are reduced, so that thetotal number of ink dots is smaller. As a result, ink dots of spot colorink tend to stand out in bright areas, with the possibility ofexacerbating graininess of the image. On the other hand, when residualink quantity RES_R is large, the hue of the reproduction color thereofis often far from that of a spot color ink. Even where such reproductioncolor is reproduced using spot color ink preferentially, ink quantitiesof the plurality of chromatic primary color inks are maintained atrelatively large values, as compared to the case when residual inkquantity RES_R is small. As a result, the total number of ink dots doesnot assume a small value, so that ink usage can be reduced withoutexacerbating graininess of the image. Accordingly, in FIGS. 26 (a) and26(b), for a given lightness parameter value Rp, it is possible toderive a larger proportion k when residual ink quantity RES_R is large.By so doing, graininess can be improved, and savings in ink usageattained.

In this Example, residual ink quantity RES_R is calculated on the basisof provisional color separation ink quantity Rp. Accordingly, wheretemporary ink quantity Rtmp determined on the basis of residual inkquantity RES_R is greater than the provisional color separation inkquantity Rp, chromatic primary color ink quantities will be smaller thanintended, with the possibility of exacerbating graininess of the imagein bright areas. Accordingly, in the example of FIGS. 26 (a) and 26(b),temporary ink quantity Rtmp is set to a value that does not exceed theprovisional color separation ink quantity Rp. By so doing, graininesscan be improved, and savings in ink usage attained. Proportion k shownin FIG. 26( b) can be thought of as corresponding to spot color inkusage rate in the present invention.

In certain instances, over a portion of the entire range of possiblevalues of residual ink quantity RES_R, the spot color ink usage rate kremains a constant value despite change of residual ink quantity RES_R.For example, in the example of FIG. 26( b), when the lightness parametervalue Rp=Re10, the spot color ink usage rate k remains a constant valueof 1 even when residual ink quantity RES_R is greater than RES_R10.Accordingly, in bright areas, the hue of reproduction color is within arange close to a chromatic primary color ink determined according toresidual ink quantity RES_R. When the lightness parameter value Rp=Rs20,the spot color ink usage rate k remains a constant value of 0 even whenresidual ink quantity RES_R is less than residual ink quantity RES_R20.Accordingly, in bright areas, the hue of reproduction color is within arange close to a spot color ink determined according to residual inkquantity RES_R. In this way, as a result of determining spot color inkusage rate k by means of attempting to improve graininess and save ink,spot color ink usage rate k may in some instances—depending on the valueof the lightness parameter value Rp—assume a constant value over aportion of the hue range. Where spot color ink usage rate k is avariable value based on hue, i.e., residual ink quantity RES_R, over atleast a part of the entire range of hue in this way, graininess can beimproved, and savings in ink usage attained.

In this Example, the spot color ink usage rate k is set to zero when thelightness parameter value Rp is equal to or less than a first valuedetermined according to residual ink quantity RES_R (in range R1 or R201in FIGS. 26 (a), 26(b)). Here, when residual ink quantity RES_R has itsmaximum possible value, the first value is preferably set larger thanzero (hereinafter referred to as first high lightness base value). By sodoing, in the highest lightness area where the lightness parameter valueRp is in a partial range of lower than the first high lightness basevalue, the spot color ink usage rate k is set to zero regardless of theresidual ink quantity RES_R, i.e., reproduction color hue. As a result,standing out of spot color ink in bright areas can be reduced, andgraininess of an image improved. In preferred practice, where the rangeof possible values for the lightness parameter value is 0% (bright) to100% (dark), such a first high lightness base value will be a brightrange of 5% or less, preferably a bright range of 10% or less, and mostpreferably a bright range of 15% or less.

Also, in this Example, when the lightness parameter value Rp is in abright range smaller than a second high lightness base value the spotcolor ink usage rate k is variable on the basis of hue, i.e. residualink quantity RES_R. Here, the second high lightness base value is asecond value when residual ink quantity RES_R is zero (not shown). Asdescribed above, in a range wherein the lightness parameter value Rp issmaller than the first high lightness base value, spot color ink usagerate k is set to zero irrespective of residual ink quantity RES_R. Inthis way, where in at least part of the entire range of the lightnessparameter value Rp (in this Example, the range smaller than the secondhigh lightness base value), spot color ink usage rate k is variable onthe basis of residual ink quantity RES_R, graininess can be improved,and savings in ink usage attained.

As regards violet ink V, temporary ink quantity Vtmp can be determinedfrom provisional color separation ink quantity Vp according to the samemethod as for red ink R.

The processes of Steps S1620, S1630 of FIG. 24 are identical to theprocesses of Steps S610, S620 of FIG. 9. That is, in Step S1620 of FIG.24, final color separation ink quantities Ro, Vo are determined fromtemporary ink quantities Rtmp, Vtmp. In Step S1630, there are determinedink quantities Co, Mo, Yo for the other inks which, in conjunction withthe final color separation ink quantity pair Ro, Vo determined in thismanner, are needed to reproduce a desired reproduction color. As aresult, a color separation ink quantity set (Co, Mo, Yo, Ro, Vo) forreproducing a desired reproduction color is determined.

In this way, in Example 1 of the color separation process in Embodiment2, final color separation ink quantity Ro, Vo of each spot color ink R,V is designed to decrease with a smaller lightness parameter value(provisional color separation ink quantity) Rp, Vp thereof, therebydecreasing the number of ink dots of spot color ink and increasing thenumber of ink dots of chromatic primary color ink in highlight areas. Aresultant advantage is that image graininess in highlight areas isimproved. In particular, since temporary ink quantities Rtmp, Vtmp areset to zero in a first range wherein provisional color separation inkquantities Rp, Vp are relatively small (in the example of FIG. 26( a),range R1 or range R201), final color separation ink quantities Ro, Vo inhighlight areas are also set to zero, and graininess of the image ismarkedly improved. It is also possible to set temporary ink quantitiesRtmp, Vtmp to some relatively small non-zero value in this first rangeR1 or R201.

Further, the spot color ink usage rate k of the final color separationink quantity Ro, Vo (FIG. 26( b)) of each spot color ink R, V isdesigned to increase with a larger residual ink quantity RES_R, RES_V,thereby improving graininess in highlight areas, as well as providingink savings.

In some instances, the tendency of ink dots to stand out (i.e. effect ongraininess) differs depending on the type of ink. Accordingly,adjustment of spot color ink usage rate depending on residual inkquantity RES_R, RES_V as in FIG. 26( a) is preferably carried outindependently for each range of hues reproducible with combinations of asingle given chromatic primary color ink and a single given spot colorink. For example, hues between cyan ink C and violet ink V arereproducible with combinations of cyan ink C and violet ink V. Colorwithin this hue range is reproduced using principally cyan ink C andviolet ink V when bright. Hues between magenta ink M and red ink R arereproducible with combinations of magenta ink M and red ink R. Bysetting usage rates of spot color inks independently depending on thecombination of ink types principally used, improved graininess accordingto the characteristics of individual inks, as well as ink savings, areattained.

D2. Examples 1-3 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 2 D2-1. Example 1 of Final Color Separation InkQuantity Calculation Process in Embodiment 2

In the final color separation ink quantity set calculation process, afinal color separation ink quantity set O is calculated using theprovisional color separation ink quantity set P for input color I. Thefinal color separation ink quantity set O is calculated so that itreproduces a color, in a range meeting ink duty limits (Condition 1-1described in Embodiment 1), which is substantially identical to a colorreproduced by the provisional color separation ink quantity set P. Inorder to reproduce color substantially identical to that of theprovisional color separation ink quantity set P, a combination of inkquantities derived by substituting some ink quantities of theprovisional color separation ink quantity set P according to thesubstitution ink quantities is used as the final color separation inkquantity set O. Here, the degree of freedom in ink substitution isgreater the larger the number of ink types used. Accordingly, in someinstances there are a plurality of ink quantity combinations possible touse as the final color separation ink quantity set O. In such instances,calculation of the final color separation ink quantity set O is carriedout with consideration to improving graininess of the image, and tosaving ink.

FIG. 27 is a flowchart depicting the processing routine for calculatinga final color separation ink quantity set in this Example. A differencefrom the process shown in FIG. 24 is that residual ink quantitiesRES_R2, RES_V2 and temporary ink quantities Rtmp, Vtmp are determined onthe basis of maximum ink quantities Rmax, Vmax which are quantities ofthe spot color inks R, V when they are used as much as possible.

In the initial Step S1300, a spot color maximum color separation inkquantity set Prmax (Crmax, Mrmax, Yrmax, Rmax, 0), Pvmax (Cvmax, Mvmax,Yvmax, 0, Vmax) for each spot color ink R, V is calculated. These spotcolor maximum color separation ink quantity sets Prmax, Pvmax are colorseparation ink quantity sets derived by substitution of some inkquantities of provisional color separation ink quantity sets P accordingto the substitution ink quantities.

The red ink R quantity in the spot color maximum color separation inkquantity set Prmax for red ink represents the maximum value assumable byred ink R quantity when ink quantities of the provisional colorseparation ink quantity set P has been substituted according to thesubstitution ink quantity. In this Example, maximum ink quantity Rmax isa maximum value that does not take ink duty limits into consideration.Accordingly, the maximum value assumable by red ink R of the final colorseparation ink quantity in actual practice may in some instances besmaller than the maximum ink quantity Rmax. A maximum value that takesink duty limits into consideration can be used instead.

Even with red ink R quantity at its maximum value Rmax in this way, insome instances there will be a plurality of available combinations ofink quantities of violet ink and the chromatic primary color inks C, M,Y. In this Example, in such instances, a combination in which inkquantity of the other spot color ink (in this Example, violet ink V) iszero is selected from among the plurality of combinations, for use asspot color maximum color separation ink quantity set Prmax.

As regards the spot color maximum color separation ink quantity setPvmax, calculation thereof is analogous to that for the spot colormaximum color separation ink quantity set Prmax.

In the next Step S1310, residual ink quantities RES_R2, RES_V2 for thespot color inks R, V are determined from the spot color maximum colorseparation ink quantity sets Prmax, Pvmax. In this Example, residual inkquantities RES_R2, RES_V2 are calculated by the following equations,using ink quantities included in the spot color maximum color separationink quantity sets Prmax, Pvmax derived in Step S1300.

RES _(—) R2=krc×Crmax+krm×Mrmax+kry×Yrmax

RES _(—) V2=kvc×Cvmax+kvm×Mvmax+kvy×Yvmax  [Equation 2-2]

In the equations, Crmax, Mrmax and Yrmax are ink quantities of thecolors CMY included in spot color maximum color separation ink quantityset Prmax; Cvmax, Mvmax and Yvmax are ink quantities of the colors CMYincluded in spot color maximum color separation ink quantity set Pvmax.Coefficients krc, krm, kry and kvc, kvm, kvy are predeterminedcoefficients for the colors CMYRV. In this way, residual ink quantityRES_R2 is the weighted sum of ink quantities other than red ink R, andresidual ink quantity RES_V2 is the weighted sum of ink quantities otherthan violet ink V. In Equation 2-1 given earlier, all ink coefficientsare 1. However, residual ink quantity can be represented by some othercoefficient for quantities of inks other than the spot color ink ofinterest. In general, residual ink quantity can be any parameter valuerelating to magnitude of quantities of inks other than the spot colorink of interest.

These coefficients krc-kry and kvc-kvy are determined according toeffect on the tendency of dots of the spot color ink to stand out. Forinks whose ink dots have high reflection density, the tendency of dotsof the spot color ink to stand out can be avoided through smaller inkquantity. Accordingly, it is preferable for the coefficient to begreater, the higher the reflection density of an ink. The smaller thedifference in color appearance from a spot color ink, the less likelydots of a spot color ink can be made to stand out with a small inkquantity, so it is preferable to set the coefficient to a larger value.

Here, comparison of reflection density of ink dots produced by aplurality of types of inks may be carried out by ejecting equalquantities of inks onto a given print medium, and measuring density witha densitometer. Comparison of color appearance may be carried out byejecting equal quantities of inks onto a given print medium, performingcalorimetric measurement of each with a calorimeter, and calculatingcolor difference (for example, color difference ΔE in the L*a*b* colorsystem) from the calorimetric results. As will be described later, thesecoefficients krc-kry and kvc-kvy can be determined on the basis of asensory test.

In the next Step S1320, temporary ink quantities Rtmp, Vtmp of the spotcolor inks R, V are determined. FIG. 28( a) is a graph of therelationship between temporary ink quantity Rtmp and maximum inkquantity Rmax. FIG. 28( b) is a graph of the relationship between theproportion k of temporary ink quantity Rtmp to maximum ink quantity Rmax(corresponds to spot color ink usage rate in the invention), and maximumink quantity Rmax.

In this Example, as shown in FIG. 28( a), temporary ink quantity Rtmp issmaller in association with smaller values of maximum ink quantity Rmax.Also, as shown in FIG. 28( b), the proportion k of temporary inkquantity Rtmp to maximum ink quantity Rmax is smaller in associationwith smaller values of maximum ink quantity Rmax. In this example, twovalues Rstart, Rend are set for the maximum ink quantity Rmax. In afirst range R11 wherein Rmax≦Rstart, the setting Rtmp=0 (k=0) is used.Over a third range R13 in which Rend≦Rmax, the setting is Rtmp=Rmax(k=1). Over a second range R12 in which Rstart<Rmax<Rend, temporary inkquantity Rtmp is set so as to change in linear fashion.

These settings in relation to temporary ink quantity Rtmp may beunderstood as follows. Spot color ink is substitutable for a pluralityof chromatic primary color inks. Thus, where spot color ink is usedpreferentially, ink quantities of the plurality of chromatic primarycolor inks according to substitution ink quantities are reduced, so thata smaller total number of ink dots is recorded onto the print medium. Onthe other hand, where chromatic primary color inks are usedpreferentially, the total number of ink dots is greater. Such differencein the number of ink dots depending on ink quantity combination isparticularly marked in instances where chromatic primary color inks maybe substituted by spot color ink, in a smaller quantity than the totalvalue of the ink quantities. Also, graininess in a reproduced area(roughness of an image) tends to stand out more the smaller the numberof ink dots. Thus, it is preferable in terms of improving imagegraininess to increase the number of ink dots in areas having smallerink quantity, i.e., fewer ink dots, by means of using chromatic primarycolor inks in place of spot color ink. In the example of FIG. 28( b),proportion k is designed to be smaller in association with smallerlevels of maximum ink quantity Rmax. Thus, by making quantity of red inkR comparable to the temporary ink quantity Rtmp, it is possible toimprove graininess in a reproduced area.

In the example of FIG. 28( a), Rtmp is zero over the first range R11.That is, in particularly bright areas, temporary ink quantity Rtmp isset to a value (namely zero) that increases the number of ink dots asmuch as possible, without the use of spot color ink. By setting thequantity of red ink R to zero in this way, standing out of dots of redink R can be avoided.

Here, the first value Rstart refers to an ink quantity such that, aslong as ink quantity is equal to or greater than this value, ink dotswill not readily stand out despite the use of red ink R. Resistance tostanding out of dots of red in R varies depending on residual inkquantity RES_R2. With larger residual ink quantity RES_R2, red ink Rdots stand out less, making it possible to increase the quantity of redink R. Accordingly, in this Example, Rstart is made smaller inassociation with larger values of RES_R2, by means of establishingRstart on the basis of the following equation.

Rstart=Crs−RES _(—) R2  [Equation 2-3]

In the equation, Crs is a predetermined constant. This constant Crs andthe coefficients krc-kry (Equation 2-2) for residual ink quantity RES_R2can be established on the basis of a sensory test such as the following,for example. First, a gradation pattern in which red ink R quantity perunit of area (hereinafter termed “index red ink quantity”) varies from 0to 100% is reproduced with a mixture of magenta ink M and yellow ink Y.Ink dots of red ink R are then printed at appropriate intervals in thepattern. In the range of small index red ink quantity, red ink R dotstend to stand out, but in the range of large index red ink quantity, redink R dots tend not to stand out. The index red ink quantity at whichdots begin to not stand out (hereinafter termed “first threshold red inkquantity”) can be used as constant Crs.

As regards the coefficients krc-kry, these may be established asfollows. A gradation pattern derived by adding a given quantity of cyanink C to a mixture of magenta ink M and yellow ink Y based on index redink quantity is reproduced. At this time, the first threshold red inkquantity is smaller in association with a larger quantity of cyan ink C.The change in first threshold red ink quantity relative to change inquantity of cyan ink C can be used as coefficient krc. As regards theother coefficients krm, kry, these may be arrived at in similar fashion,using gradation patterns derived by adding a given quantity of each ink.

In the example of FIGS. 28( a) and 28(b), the setting Rtmp=Rmax is usedin the third range R13. That is, over a range in which ink quantity isparticularly large, temporary ink quantity Rtmp is set to a value thatpreferentially uses red ink R (i.e., the maximum ink quantity Rmax). Bysetting the quantity for red ink R to a large value in this way, totalink quantity used can be minimized. As a result, savings in quantity ofink used are provided, and wrinkling of print media can be avoided.

Here, the second value Rend means an ink quantity such that, as long asink quantity is equal to or greater than this value, there begins to beno observable difference between graininess produced by red ink R andgraininess produced by mixtures of other inks. In bright areas or highsaturation areas, even if red ink R quantity is set to a large value,the red ink dots do not readily stand out owing to the large number ofdots. Where residual ink quantity RES_R is large, dots of red ink R areeven less likely to stand out, making it possible to further increasethe ink quantity of red ink R. Accordingly, in this Example, Rend ismade smaller in association with larger values of RES_R2, by means ofestablishing Rend on the basis of the following equation.

Rend=Cre−RES _(—) R2  [Equation 2-4]

In the equation, Cre is a predetermined constant. This constant Cre canbe established on the basis of a sensory test such as the following, forexample. First, a gradation pattern in which red ink R quantity per unitof area (hereinafter termed “index red ink quantity”) varies from 0 to100% is reproduced with a mixture of magenta ink M and yellow ink Y. Insimilar fashion, a gradation pattern in which index red ink quantityvaries from 0 to 100% is reproduced using red ink R. In the range ofsmall index red ink quantity, the gradation pattern produced by thecolor mixture has more ink dots and accordingly better graininess. Inthe range of large index red ink quantity, each of the two gradationpatterns has a large number of ink dots, so the difference in graininessstarts to disappear. The index red ink quantity at which difference ingraininess apparent from comparison of the two gradation patterns beginsto disappear (hereinafter termed “second threshold red ink quantity”)can be used as constant Cre.

As regards the coefficients krc-kry for residual ink quantity RES_R2,these may be established as follows. When determining coefficient krc,for example, a gradation pattern is printed with a given additionalquantity of cyan ink C added to a mixture of magenta ink M and yellowink Y corresponding to index red ink quantity variation, and anothergradation pattern is also printed with the same additional quantity ofcyan ink C to the red ink R quantities of the index red ink quantityvariation. When the patterns are compared, the second threshold red inkquantity is smaller, the greater the quantity of added ink. The changein second threshold red ink quantity relative to change in quantity ofadded ink can be used as coefficient krc. As regards the othercoefficients krm, kry, these may be arrived at in similar fashion.

In this Example, coefficients krc-kry, i.e. residual ink quantityRES_R2, are used in common for the first and second values Rstart, Rend.Where effects of inks C, M, Y on first and second values Rstart, Renddiffer, it is preferable to set residual ink quantity RES_R2independently for the first and second values Rstart, Rend.

Where a particular ink having minimal effect on the first and secondvalues Rstart, Rend, that is, ink that, despite being increased inquantity, attains no reduction in the tendency of spot color ink dots tostand out, it is preferable to set the coefficient for that particularink to zero. For example, where the reflection density of ink dots ofyellow ink Y is low and, despite being increased in quantity, thetendency of red ink R dots to stand out is unchanged, it is preferableto set the coefficient kry for yellow ink Y to zero. By so doing, evenif reproduction color hue is close to yellow ink Y, since the usage rateof red ink R (proportion k) is not large, standing out of dots of redink R can be avoided. Where reproduction color hue is close to magentaink M, since the usage rate of red ink R (proportion k) is set to arelatively large value, ink savings are attained. Similarly, where thetendency of violet ink V dots to stand out is unchanged despiteincreasing the ink quantity of yellow ink K, coefficient kry can be setto zero. When reproduction color hue is relatively close to cyan ink Cor magenta ink M, since the usage rate of violet ink V is set to arelatively large value, ink savings are attained. It is also possible toset kry, kvy to relatively small, non-zero values.

By setting first and second values Rstart, Rend in the above manner, theusage rate k for a particular maximum ink quantity Rmax is relativelylarge when the residual ink quantity RES_R2 is relatively large.Further, residual ink quantity RES_R2 is determined in consideration ofink characteristics. Accordingly, graininess can be improvedappropriately depending on ink characteristics. When residual inkquantity RES_R2 is relatively large, since usage rate k becomesrelatively large, ink quantity of spot color ink can be increased, andquantities of the plurality of chromatic primary color inks can bereduced. Accordingly, ink savings can be achieved appropriatelydepending on ink characteristics. In this Example, usage rate k iscalculated on the basis of maximum ink quantity when spot color ink isused as much as possible. Accordingly, the available range of spot colorink quantity can be utilized effectively.

Maximum ink quantity Rmax may also be thought of as an index valueindicative of lightness of input color I, that is, a lightness parametervalue correlated to lightness of reproduction color. In a bright area,quantity of each ink for reproducing the color is small. Accordingly,the maximum ink quantity Rmax of red ink R is small as well. In a darkarea, on the other hand, quantity of each ink for reproducing the coloris greater. Accordingly, in an area in which red ink R is used, themaximum ink quantity Rmax thereof will be greater. That is, a smallervalue of maximum ink quantity Rmax indicates a brighter area, and alarger value indicates a darker area. Accordingly, temporary inkquantity Rtmp and proportion k in FIG. 27 may be thought of as being setso as to be smaller with greater lightness of input color I.

Temporary ink quantity Rtmp is designed to vary continuously over theentire range for maximum ink quantity Rmax. By so doing, in gradationareas over which ink quantity varies continuously, sharp changes inquantity of each color ink, and resulting standing out of boundaries,can be avoided. Temporary ink quantity Rtmp is not limited to anarrangement whereby it varies in linear fashion with respect to maximumink quantity Rmax; it could instead be designed to vary smoothly using acurve, for example.

While FIGS. 28( a) and 28(b) show calculation of temporary ink quantityRtmp of red ink R, temporary ink quantity of other types of spot colorink could be calculated in the same manner. In any case, the design issuch that the proportion of temporary ink quantity to maximum inkquantity decreases with smaller maximum ink quantity. By so doing, thenumber of ink dots can be increased by preferentially using chromaticprimary color ink in place of spot color ink, so that graininess can beimproved.

Here, it is preferable to design the spot color ink usage rate so as tobe smaller when residual ink quantity is relatively small. By so doing,graininess can be improved, and ink savings attained.

The processes of Steps S1330, S1340 in FIG. 27 are the same as theprocesses of Steps S310, S320 of FIG. 15. That is, in Step S1330 of FIG.27, calculation of final color separation ink quantities Ro, Vo for spotcolor inks R, V is performed on the basis of temporary ink quantitiesRtmp, Vtmp of spot color inks R, V established in the aforementionedStep S1320.

As described hereinabove, the final color separation ink quantity setcalculation process of this Example is carried out with consideration toimproving graininess. Where an image area to be reproduced is bright,quantity of each color of ink is small. Since less spot color ink isused and more chromatic primary color ink is used in such area,graininess can be improved.

In an area of particularly high lightness, each quantity of an ink setis much smaller relative to ink duty limits. Accordingly, ink dutylimits can still be met with a combination of greater ink quantities. Asa result, where temporary ink quantities Rtmp, Vtmp of the spot colorinks are zero, final color separation ink quantities Ro, Vo can bebrought to zero. That is, in an area of particularly high lightness,color is reproduced without the use of spot color inks, and thus in suchparticularly bright areas, standing out of ink dots of spot color inkcan be avoided.

When residual ink quantities RES_R2, RES_V2 are relatively small,quantities of spot color inks R, V become relatively small; and whenresidual ink quantities RES_R2, RES_V2 are relatively large, quantitiesof spot color inks R, V become relatively large. Accordingly, it ispossible to improve graininess while attaining ink savings, as well aspreventing wrinkling of print media.

In this Example, the final color separation ink quantity set calculationprocess is performed according to the temporary ink quantity Rtmp, Vtmpestablished independently for each of the spot color inks, that is, tothe effect of each ink on graininess. As a result, graininess may beimproved appropriately in consideration of the effect of each spot colorink.

This sort of final color separation ink quantity set calculation processmay be carried out in similar fashion even where more types of spotcolor ink are used. For example, where three spot color inks are used,first, a temporary ink quantity is calculated for each ink. Next, acombination of ink quantities represented by a point situated within apermissible range (represented by a solid in a color space in which thethree ink quantities are represented by base vectors) and close to apoint represented by a temporary ink quantity set is used as a finalcolor separation ink quantity set of spot color ink. In this case aswell, where a final color separation ink quantity set is calculatedaccording to the positional relationship of the permissible range and astraight line passing through a point represented by a temporary inkquantity set, the final color separation ink quantity set can becalculated in consideration of the balance of effect of each spot colorink on graininess.

As described hereinabove, the final color separation ink quantity setcalculation process of this Example is carried out in consideration ofimproving graininess, whereby roughness of images in bright areasthereof can be reduced.

D2-2. Example 2 of Final Color Separation Ink Quantity CalculationProcess in Embodiment 2

Example 2 is different from the above Example 1 in that temporary inkquantity Rtmp is established depending on lightness L of reproductioncolor. FIG. 29 is a graph of the relationship between lightness L, andthe proportion k of temporary ink quantity Rtmp versus maximum inkquantity Rmax.

When provisional color separation ink quantity set P is expressed in aprimary color space (FIG. 13( a)), the distance between a point producedby projecting point P onto the achromatic color axis (the straight lineconnecting the origin W and a point K) and the origin W may be used aslightness L of the reproduction color. Here, greater distance from theorigin W indicates lower lightness. As the maximum ink quantity Rmax,there may be used the maximum value assumable by the quantity of red inkR, among combinations of ink quantities able to reproduce a colorsubstantially identical to the color to be reproduced by provisionalcolor separation ink quantity set P.

In this Example, as shown in FIG. 29, the proportion k of temporary inkquantity Rtmp to maximum ink quantity Rmax is set so as to be smallerthe higher the lightness L of the reproduction color is, i.e. thebrighter it is. Temporary ink quantity for the other spot color ink isset similarly. Accordingly, in an image area of high lightness L andsmall ink quantities of each ink, less spot color ink is used, andchromatic primary color ink is used preferentially to increase thenumber of ink dots, so that graininess can be improved.

In the brightest range R21 wherein Lstart≦L, the setting Rtmp=0 is used.As a result, in areas of particularly high lightness, colors arereproduced without spot color ink, so that standing out of ink dots ofspot color ink can be prevented. In the darkest range R23 whereinL≦Lend, the setting Rtmp=Rmax is used. As a result, in areas ofparticularly low lightness, colors are reproduced using spot color inkpreferentially, thereby providing savings in quantity of ink used, andalso preventing wrinkling of the print medium.

The values Lstart, Lend for lightness L can be set in the same manner asRstart, Rend in FIG. 28( a) described previously. For example, they canbe set on the basis of a sensory test by comparing a spot color inkgradation pattern reproduced while varying lightness L from minimum tomaximum value, with a gradation pattern produced by a mixture ofchromatic primary color inks.

It is preferable to establish the first and second values Lstart, Lendso that they are larger the larger the value of residual ink quantityRES_R2 is. By so doing, graininess can be improved and ink savingsattained. Preferably, Lstart, Lend will be established independently foreach hue range reproducible by a combination of a single chromaticprimary color ink and a single spot color ink. By so doing, graininesscan be improved and ink savings attained, in an appropriate mannerdepending on hue.

In this Example, as regards violet ink V, temporary ink quantity Vtmpdepending on lightness L is established using the same method as for redink R. Where even more spot color inks are used, temporary inkquantities can be established in similar fashion.

As the final color separation ink quantity set calculation process ofthe color separation process of this Example, the method employed in thefinal color separation ink quantity set calculation process S1520 of thecolor separation process shown in FIG. 23 could be used instead of thevarious methods depicted in FIGS. 27-28 and FIG. 29. Similarly, as thecolor separation ink quantity set calculation process S1520 of the colorseparation process shown in FIG. 23, any of the various methods (FIGS.27-28, FIG. 29) employed in the final color separation ink quantity setcalculation process of this Example could be used.

D3. Other Example of Color Separation Process in Embodiment 2

Example 2 and Example 3 of the color separation process in Embodiment 1described hereinabove may also be applied to Embodiment 2. However, whenapplying Example 3 of the color separation process in Embodiment 1 toEmbodiment 2, it is preferable to add ink quantity of black ink K toresidual ink quantity. For example, with regard to the residual inkquantities RES_R2, RES_V2 given in Equation 2-2 hereinabove, quantity ofblack ink K multiplied by a coefficient may be added in the same manneras the chromatic primary color inks C, M, Y. The coefficient for blackink K can be determined analogously to coefficients for other inks. Inthis way, by determining ink quantities of the spot color inks R, Vtaking into consideration as well the quantity of black ink K, imagegraininess can be improved, and ink savings attained.

When the color separation process of this Example is implemented in StepS20 of the color conversion lookup table creation process illustrated inFIG. 6, the second tone value set consists quantities of the chromaticprimary color inks CMY, spot color inks RV, and black ink K. Thus, inStep S30, color patches reproduced using the colors CMYRVK are created.

E. Ink Set Variations

In the Examples described hereinabove, ink sets of various kinds otherthan the ink set depicted in FIGS. 5( a)-5(c) can be employed. FIGS.30-37 illustrate ink components in additional Examples of suchemployable ink sets. Black ink K components and components of othercolor inks except their colorants are the same as in FIG. 5( a), andthey are therefore not shown in the drawings. Differences from the inkset in FIG. 5( a) lie in partially different colorant type and density.As a result, these ink sets can improve reproduction of colors that areslightly different from one another. Accordingly, by selecting an inkset appropriate for the image it is desired to print, a printed resultof higher picture quality can be obtained.

The ink sets of FIGS. 30-45 include substitution ink quantities for redink R and violet ink V respectively, derived by colorimetric measurementof color patches. In this way, total substitution ink quantity in eachof these ink sets is 1.7 or greater. As a result, it is possible toreproduce higher saturation by using spot color ink in comparablequantity to a mixture of chromatic primary color inks. As a result, byutilizing spot color ink and chromatic primary color ink, it is possibleto reproduce a wider range of color than the area reproducible withchromatic primary color ink only.

Inks are not limited to the formulations shown in FIG. 5 and FIGS. 30-37hereinabove; appropriate inks prepared according to other formulationsmay be used as well. As regards the color and number of inks as well,these are not limited to the combinations hereinabove; for example, anarrangement wherein red ink R alone can be utilized as the spot colorink, or an arrangement wherein green ink or blue ink is used as a spotcolor ink is acceptable as well. However, it is preferable to use inkswhich in combination with one another can reproduce achromatic color aschromatic primary color inks, and to use inks having hue different fromany of the chromatic primary color inks as spot color inks. By using inksets composed in this manner, it is possible to execute color separationwith consideration to extending the color reproduction range.

As described hereinabove, in the preceding Examples, the final colorseparation ink quantity set calculation process is carried out withconsideration to improving graininess, thereby reducing roughness of animage in bright areas thereof. Additionally, since quantity of spotcolor ink is determined on the basis of residual ink quantities,roughness of an image in bright areas thereof can be reduced, and inksavings attained. By using chromatic primary color ink and spot colorink, the color separation process can be performed on the basis ofextended chromatic color of high tone value. Accordingly, it is a simplematter to perform a color separation process that takes intoconsideration an expanded color reproduction range.

The final color separation ink quantity set calculation process is notlimited to the processes described hereinabove; generally, any processwould be acceptable as far as it adjusts each ink quantity of the colorseparation ink quantity set for reproducing a reproduction color suchthat quantity of spot color ink decreases at a rate of change greaterthan a rate of change of a lightness parameter when the lightnessparameter changes in a lighter direction.

F. Variations

The present invention is not limited to the Examples or embodiments setforth hereinabove, and may be reduced to practice in various other modeswithout departing from the scope and spirit thereof. The followingvariations are possible, for example.

F1 Variation 1

In the preceding Examples, the hues of the inks of a useable ink set aredifferent from one another; however, an arrangement wherein a pluralityof types of ink having substantially identical hue but differentdensities are useable would be acceptable as well. In this case, byusing inks of different densities depending on tone value of a hue, itis possible to improve graininess (image roughness) that tends to standout more when there are small numbers ink dots, as well as to reducebanding (striped patterns) that tends to stand out more when there arelarge numbers of ink dots. In this case, quantity of each ink may becalculated using a so-called linear programming method, takingquantities for all inks into consideration when establishing conditionssuch as the aforementioned ink duty limits and substitution inkquantities. A method of calculating color separation ink quantity on ahue-by-hue basis and reassigning ink quantities so derived to aplurality of inks of substantially identical hue but different densitieswould also be acceptable. In this case as well, it is preferable toestablish ink duty limits that take into consideration quantities of allinks, and to ensure that final ink quantities of the inks meet the inkduty limits.

In the preceding Examples, “ink quantity” refers to tone value of eachink representing a range of 0%-100% (where ink quantity for reproducinga solid area is 100%), which is output from a color conversion lookuptable. Where a plurality of types of ink of substantially identical huebut different densities are useable, color separation can beaccomplished by making the total value of colorant of lighter and darkerinks having the same hue correspond to “ink quantity.” By assigning “inkquantities” derived thusly to lighter and darker inks, appropriate colorcan be reproduced.

F2. Variation 2

The present invention is applicable also to thermal transfer printers ordrum scan printers. The invention may be implemented not only inso-called ink jet printers, but more generally in printing devices thatreproduce color by means of mixing inks of several colors. Such printingdevices include, for example, fax machines and copy machines.

G3. Variation 3

Various values can be used as a lightness parameter value. In theExamples of color separation processes shown in FIGS. 10( a)-10(b) andFIGS. 26( a)-26(b), provisional color separation ink quantity of spotcolor ink is used as a lightness parameter value. In these Examples,there is determined a provisional color separation ink quantity set suchthat input color and reproduction color match, and reproduction color isreproduced under the condition of minimized total ink quantity. In theExamples of the final color separation ink quantity set calculationprocesses shown in FIGS. 16( a)-16(b) and FIGS. 28( a)-28(b), maximumink quantity of spot color ink is used as a lightness parameter value.This maximum ink quantity represents the maximum value of ink quantitydetermined from the reproduction color associated with an input color,and is the maximum value of quantity of spot color ink amongcombinations of ink quantities that reproduce color substantiallyidentical to that of the provisional color separation ink quantity setfor reproducing reproduction color. Generally, any value that representsink quantity of spot color ink determined from input color orreproduction color according to a predetermined condition can be used asa lightness parameter value. Lightness parameter values are not limitedto those representing ink quantity of spot color ink, but could insteadbe values correlated with lightness of reproduction color. For example,lightness of reproduction color associated with input color, as in theExamples of the final color separation ink quantity set calculationprocesses shown in FIGS. 18, 20, and 29, can be used as a lightnessparameter value.

Within the range of a lightness parameter value, the brightest rangewherein spot color ink quantity is adjusted to zero will preferablyconstitute the brightest range of 5% or less, preferably the brightestrange of 10% or less, and most preferably the brightest range of 15% orless, where the range of possible values for the lightness parametervalue is 0% (bright) to 100% (dark).

F4. Variation 4

In the preceding Examples, there were described cases in which spotcolor ink quantity is determined on the basis of residual ink quantity;however, it could instead by determined on the basis of a parameterhaving correlation with reproduction color hue. In this case, when hueis relatively close to a spot color ink, ink quantities are determinedsuch that the usage rate of the spot color ink is relatively high, orwhen hue is relatively close to a chromatic primary color ink, inkquantities are determined such that the usage rate of the spot color inkis relatively low. By so doing, graininess can be improved appropriatelydepending on hue, and ink savings can be attained. Various values can beused as parameters having correlation with reproduction color hue. Forexample, in the primary color space depicted in FIGS. 13( a)-13(c),where the straight line connecting the origin W and point K is termedthe achromatic color line and a vertical projection of a particularpoint in the primary color space onto the achromatic color line istermed the projected point, the direction from the projected point tothe particular point in the primary color space can be used as aparameter value having correlation to hue.

F5. Variation 5

In the preceding Examples, some arrangements realized through hardwarecould instead by realized by software, and conversely some arrangementsrealized through software could instead by realized by hardware. Forexample, it would be possible for some functions of printer driver 96(FIG. 1) to be executed instead by the control circuit 40 (FIG. 3)inside the printer 20.

While the present invention has been shown and described in detailhereinabove, the descriptions are merely exemplary, and should not beconstrued as limiting the technical scope of the invention, which islimited only by the appended Claims.

1. A printing apparatus comprising: a print controller that performsprinting of an image on a print medium using an ink set, the ink setincluding a plurality of chromatic primary color inks and at least onespot color ink, the plurality of chromatic primary color inks beingcapable of reproducing achromatic color when used in combination, theplurality of chromatic primary color inks including a first ink and asecond ink, the spot color ink having a hue different from any of theplurality of chromatic primary color inks, the hue of the spot color inkbeing capable of being reproduced by a combination of the first ink andthe second ink, wherein the print controller performs printing of afirst color without the spot color ink, the first color having apredetermined hue between a hue of the first ink and a hue of the spotcolor ink, the first color having a predetermined lightness and apredetermined saturation, and the print controller performs printing ofa second color with the spot color ink, the second color having apredetermined hue being closer to the hue of the first ink than the hueof the first color is, the second color having the same lightness andthe same saturation as the first color.
 2. A printing apparatuscomprising: a print controller that performs printing of an image on aprint medium using an ink set, the ink set including a plurality ofchromatic primary color inks and at least one spot color ink, theplurality of chromatic primary color inks being capable of reproducingachromatic color when used in combination, the plurality of chromaticprimary color inks including a first ink and a second ink, the spotcolor ink having a hue different from any of the plurality of chromaticprimary color inks, the hue of the spot color ink being capable of beingreproduced by a combination of the first ink and the second ink, whereinin a case where the print controller performs printing of a plurality offirst colors having a first hue between a hue of the first ink and a hueof the spot color ink, a predetermined saturation and mutually differentlightness: the print controller performs printing of the first colorlighter than a first lightness with the spot color ink; and the printcontroller performs printing of the first color darker than the firstlightness with the spot color ink, and in a case where the printcontroller performs printing of a plurality of second colors having asecond hue being closer to the hue of the first ink than the first hueis, the predetermined saturation and mutually different lightness: theprint controller performs printing of the second color lighter than asecond lightness lighter than the first lightness without the spot colorink; and the print controller performs printing of the second colordarker than the second lightness with the spot color ink.
 3. A printingapparatus comprising: a print controller that performs printing of animage on a print medium using an ink set, the ink set including aplurality of chromatic primary color inks and at least one spot colorink, the plurality of chromatic primary color inks being capable ofreproducing achromatic color when used in combination, the plurality ofchromatic primary color inks including a first ink and a second ink, thespot color ink having a hue different from any of the plurality ofchromatic primary color inks, the hue of the spot color ink beingcapable of being reproduced by a combination of the first ink and thesecond ink, wherein in a case where the print controller performsprinting of a plurality of first colors having a first hue between a hueof the first ink and a hue of the spot color ink, a predeterminedsaturation and mutually different lightness: the print controllerdecreases an ink quantity of the spot color ink at a first variation aslightness becomes higher to perform printing of the first color lighterthan a first lightness; and the print controller decreases the inkquantity of the spot color ink at a second variation as lightnessbecomes higher to perform printing of the first color darker than thefirst lightness, the second variation being smaller than the firstvariation, in a case where the print controller performs printing of aplurality of second colors having a second hue being closer to the hueof the first ink than the first hue is, the predetermined saturation andmutually different lightness: the print controller decreases the inkquantity of the spot color ink at a third variation as lightness becomeshigher to perform printing of the second color lighter than a secondlightness lighter than the first lightness; and the print controllerdecreases the ink quantity of the spot color ink at a fourth variationas lightness becomes higher to perform printing of the second colordarker than the second lightness, the fourth variation being smallerthan the third variation.
 4. A printing apparatus comprising: a printcontroller that performs printing of an image on a print medium using anink set, the ink set including a plurality of chromatic primary colorinks and at least one spot color ink, the plurality of chromatic primarycolor inks and at least one spot color ink, the plurality of chromaticprimary color inks being capable of reproducing achromatic color whenused in combination, the plurality of chromatic primary color inksincluding a first ink and a second ink, the spot color ink having a huedifferent from any of the plurality of chromatic primary color inks, thehue of the spot color ink being capable of being reproduced by acombination of the first ink and the second ink, wherein the printcontroller performs printing of a first color with a first quantity ofthe second ink, the first color having a first hue between a hue of thefirst ink and a hue of the spot color ink, and the print controllerperforms printing of a second color with a second quantity of the secondink, the second color having a second hue being closer to the hue of thefirst ink than the hue of the first color is, the second quantity beingsmaller than the first quantity.
 5. A printing apparatus comprising: aprint controller that performs printing of an image on a print mediumusing an ink set, the ink set including a plurality of chromatic primarycolor inks and at least one spot color ink, the plurality of chromaticprimary color inks being capable of reproducing achromatic color whenused in combination, the plurality of chromatic primary color inksincluding a first ink and a second ink, the spot color ink having a huedifferent from any of the plurality of chromatic primary color inks, thehue of the spot color ink being capable of being reproduced by acombination of the first ink and the second ink, wherein a difference ofan ink quantity of the second ink is greater than a difference of an inkquantity of the spot color ink between printing of a first color andprinting of a second color, the first color having a first hue between ahue of the first ink and a hue of the spot color ink, the second colorhaving a second hue being closer to the hue of the first ink than thefirst hue is.